diff --git a/src/audio_core/renderer/command/effect/biquad_filter.cpp b/src/audio_core/renderer/command/effect/biquad_filter.cpp
index d40fa71b7..e900a0e28 100644
--- a/src/audio_core/renderer/command/effect/biquad_filter.cpp
+++ b/src/audio_core/renderer/command/effect/biquad_filter.cpp
@@ -8,7 +8,8 @@
 
 namespace AudioCore::Renderer {
 /**
- * Biquad filter float implementation.
+ * Biquad filter float implementation (Direct Form 2).
+ * This matches Ryujinx's implementation for better numerical stability.
  *
  * @param output       - Output container for filtered samples.
  * @param input        - Input container for samples to be filtered.
@@ -27,29 +28,33 @@ void ApplyBiquadFilterFloat(std::span<s32> output, std::span<const s32> input,
                          Common::FixedPoint<50, 14>::from_base(b_[2]).to_double()};
     std::array<f64, 2> a{Common::FixedPoint<50, 14>::from_base(a_[0]).to_double(),
                          Common::FixedPoint<50, 14>::from_base(a_[1]).to_double()};
-    std::array<f64, 4> s{Common::BitCast<f64>(state.s0), Common::BitCast<f64>(state.s1),
-                         Common::BitCast<f64>(state.s2), Common::BitCast<f64>(state.s3)};
+
+    // Direct Form 2 uses only 2 state variables (s0, s1)
+    // s2 and s3 are unused in Direct Form 2
+    f64 s0{Common::BitCast<f64>(state.s0)};
+    f64 s1{Common::BitCast<f64>(state.s1)};
 
     for (u32 i = 0; i < sample_count; i++) {
         f64 in_sample{static_cast<f64>(input[i])};
-        auto sample{in_sample * b[0] + s[0] * b[1] + s[1] * b[2] + s[2] * a[0] + s[3] * a[1]};
+        f64 sample{in_sample * b[0] + s0};
 
         output[i] = static_cast<s32>(std::clamp(sample, min, max));
 
-        s[1] = s[0];
-        s[0] = in_sample;
-        s[3] = s[2];
-        s[2] = sample;
+        // Update state using Direct Form 2
+        s0 = in_sample * b[1] + sample * a[0] + s1;
+        s1 = in_sample * b[2] + sample * a[1];
     }
 
-    state.s0 = Common::BitCast<s64>(s[0]);
-    state.s1 = Common::BitCast<s64>(s[1]);
-    state.s2 = Common::BitCast<s64>(s[2]);
-    state.s3 = Common::BitCast<s64>(s[3]);
+    state.s0 = Common::BitCast<s64>(s0);
+    state.s1 = Common::BitCast<s64>(s1);
+    // s2 and s3 are unused in Direct Form 2, but we keep them zeroed for consistency
+    state.s2 = 0;
+    state.s3 = 0;
 }
 
 /**
  * Biquad filter float implementation with native float coefficients (SDK REV15+).
+ * Uses Direct Form 2 for better numerical stability, matching Ryujinx.
  */
 void ApplyBiquadFilterFloat2(std::span<s32> output, std::span<const s32> input,
                              std::array<f32, 3>& b, std::array<f32, 2>& a,
@@ -59,26 +64,28 @@ void ApplyBiquadFilterFloat2(std::span<s32> output, std::span<const s32> input,
 
     std::array<f64, 3> b_double{static_cast<f64>(b[0]), static_cast<f64>(b[1]), static_cast<f64>(b[2])};
     std::array<f64, 2> a_double{static_cast<f64>(a[0]), static_cast<f64>(a[1])};
-    std::array<f64, 4> s{Common::BitCast<f64>(state.s0), Common::BitCast<f64>(state.s1),
-                         Common::BitCast<f64>(state.s2), Common::BitCast<f64>(state.s3)};
+
+    // Direct Form 2 uses only 2 state variables (s0, s1)
+    // s2 and s3 are unused in Direct Form 2
+    f64 s0{Common::BitCast<f64>(state.s0)};
+    f64 s1{Common::BitCast<f64>(state.s1)};
 
     for (u32 i = 0; i < sample_count; i++) {
         f64 in_sample{static_cast<f64>(input[i])};
-        auto sample{in_sample * b_double[0] + s[0] * b_double[1] + s[1] * b_double[2] +
-                    s[2] * a_double[0] + s[3] * a_double[1]};
+        f64 sample{in_sample * b_double[0] + s0};
 
         output[i] = static_cast<s32>(std::clamp(sample, min, max));
 
-        s[1] = s[0];
-        s[0] = in_sample;
-        s[3] = s[2];
-        s[2] = sample;
+        // Update state using Direct Form 2
+        s0 = in_sample * b_double[1] + sample * a_double[0] + s1;
+        s1 = in_sample * b_double[2] + sample * a_double[1];
     }
 
-    state.s0 = Common::BitCast<s64>(s[0]);
-    state.s1 = Common::BitCast<s64>(s[1]);
-    state.s2 = Common::BitCast<s64>(s[2]);
-    state.s3 = Common::BitCast<s64>(s[3]);
+    state.s0 = Common::BitCast<s64>(s0);
+    state.s1 = Common::BitCast<s64>(s1);
+    // s2 and s3 are unused in Direct Form 2, but we keep them zeroed for consistency
+    state.s2 = 0;
+    state.s3 = 0;
 }
 
 /**
@@ -117,15 +124,52 @@ void BiquadFilterCommand::Dump(
 }
 
 void BiquadFilterCommand::Process(const AudioRenderer::CommandListProcessor& processor) {
+    if (state == 0) {
+        LOG_ERROR(Service_Audio, "BiquadFilterCommand: Invalid state pointer (null)");
+        return;
+    }
+
     auto state_{reinterpret_cast<VoiceState::BiquadFilterState*>(state)};
     if (needs_init) {
         *state_ = {};
     }
 
+    // Validate buffer indices and bounds
+    if (input < 0 || processor.sample_count == 0) {
+        LOG_ERROR(Service_Audio,
+                  "BiquadFilterCommand: Invalid input buffer index or sample count - input={}, "
+                  "sample_count={}",
+                  input, processor.sample_count);
+        return;
+    }
+
+    // If output is invalid but input is valid, use input as output (in-place processing)
+    s16 effective_output = output;
+    if (output < 0) {
+        LOG_WARNING(Service_Audio,
+                    "BiquadFilterCommand: Invalid output buffer index ({}), using input ({}) for "
+                    "in-place processing",
+                    output, input);
+        effective_output = input;
+    }
+
+    const u64 input_offset = static_cast<u64>(input) * processor.sample_count;
+    const u64 output_offset = static_cast<u64>(effective_output) * processor.sample_count;
+
+    if (input_offset + processor.sample_count > processor.mix_buffers.size() ||
+        output_offset + processor.sample_count > processor.mix_buffers.size()) {
+        LOG_ERROR(Service_Audio,
+                  "BiquadFilterCommand: Buffer indices out of bounds - input_offset={}, "
+                  "output_offset={}, sample_count={}, buffer_size={}",
+                  input_offset, output_offset, processor.sample_count,
+                  processor.mix_buffers.size());
+        return;
+    }
+
     auto input_buffer{
-        processor.mix_buffers.subspan(input * processor.sample_count, processor.sample_count)};
+        processor.mix_buffers.subspan(input_offset, processor.sample_count)};
     auto output_buffer{
-        processor.mix_buffers.subspan(output * processor.sample_count, processor.sample_count)};
+        processor.mix_buffers.subspan(output_offset, processor.sample_count)};
 
     if (use_float_processing) {
         // REV15+: Use native float coefficients if available
@@ -143,6 +187,44 @@ void BiquadFilterCommand::Process(const AudioRenderer::CommandListProcessor& pro
 }
 
 bool BiquadFilterCommand::Verify(const AudioRenderer::CommandListProcessor& processor) {
+    // Validate state pointer
+    if (state == 0) {
+        LOG_ERROR(Service_Audio, "BiquadFilterCommand: Invalid state pointer (null)");
+        return false;
+    }
+
+    // Validate input buffer index (required)
+    if (input < 0) {
+        LOG_ERROR(Service_Audio, "BiquadFilterCommand: Invalid input buffer index - input={}", input);
+        return false;
+    }
+
+    // Output can be invalid - we'll handle it by using input as output (in-place processing)
+    // So we don't fail verification if only output is invalid
+    s16 effective_output = output;
+    if (output < 0) {
+        effective_output = input;
+    }
+
+    if (processor.sample_count == 0) {
+        LOG_ERROR(Service_Audio, "BiquadFilterCommand: Invalid sample count - sample_count={}",
+                  processor.sample_count);
+        return false;
+    }
+
+    const u64 input_offset = static_cast<u64>(input) * processor.sample_count;
+    const u64 output_offset = static_cast<u64>(effective_output) * processor.sample_count;
+
+    if (input_offset + processor.sample_count > processor.mix_buffers.size() ||
+        output_offset + processor.sample_count > processor.mix_buffers.size()) {
+        LOG_ERROR(Service_Audio,
+                  "BiquadFilterCommand: Buffer indices out of bounds - input_offset={}, "
+                  "output_offset={}, sample_count={}, buffer_size={}",
+                  input_offset, output_offset, processor.sample_count,
+                  processor.mix_buffers.size());
+        return false;
+    }
+
     return true;
 }