That’s a good question. I think for now, in loopback, there probably wouldn’t be much difference now that the errant 2 kHz that was showing up in the notch is gone as that limited the performance. But, once you stick a really high-powered class D DUT in the loop, maybe it would help. Specifically, there are some really high performance class D amps with that are hitting -116 dB THDN and with 131 dB of dynamic range that could conceivably demonstrate the need for additional decoupling.
I wrote a short study on the differences between the common high power class D (such as TPA3255) and this next generation class D (Purifi 1ET400A) HERE. The study looks at noise floor. If there was a situation that might demonstrate the need for more isolation, it’d be that. That amp runs at a 500 kHz switching frequency, so the ~90 dB of attenuation the QA451 has at 500 kHz would help. But I don’t know if it would be enough. The plot below is the SPICE sim of the QA452 filter (from HERE).
