Before I added the RF isolation treatment on the M scaler outputs, HF GHz clip on ferrites sounded considerably better - warmer sound quality, with better depth and focus. I understand why it works as the problem is down to RF currents (peak problem area is around 2 GHz) flowing in a loop from the mains>Hugo M scaler FPGA ground>galvanic isolated OP>BNC cable> DAC ground plane> back to the mains. It's when the current flowing through the DAC ground plane is where the SQ problems are happening. If we can reduce the value of the currents by increasing the loop impedance then we get less voltage drops across the DAC ground plane and then less correlated noise and lower noise floor modulation. The lower noise floor modulation improves warmth; the reduction in correlated noise (that's noise/distortion that is in the audio bandwidth and is linked to the actual signal) degrades small signal amplitude accuracy, and that degrades the perception of depth.
It's all very straightforward and well understood - double the loop impedance at 2GHz and currents will half, and we get a corresponding improvement in sound quality - better depth and a warmer sound.
It doesn't matter too where you increase the loop impedance either; you can do it at the BNC output drivers, or through the cable, or via the PSU on the M scaler, or via the PSU on the DAC - it's the total loop impedance that counts. If the loop impedance is infinite, no current can flow, audio nirvana results. Problem is, at 2GHz even 2pF of capacitance (which is a very small capacitance) is very significant - it is 40 ohms, so will allow current to flow. Moreover, the ear/brain is ultra sensitive to this problem, so the faintest trace of an error is audible.
So why did adding ferrites improve the SQ? Ferrites work by increasing the common mode impedance, but keeping the differential impedance the same. The wanted signal is differential, the unwanted current is common mode; so adding ferrites increases the loop impedance, without affecting the wanted signal; if you double the number of ferrites, you half the loop currents, thus halving the SQ problem.
The treatments I put into the M scaler was extensive and aimed at increasing the loop impedance, by improving the galvanic isolation, improving the drivers and the driver PSU, and adding RF filters and chip ferrites into the BNC output ground. In terms of sound quality, I gained about an order of magnitude improvement in SQ - it went from a big problem to a small one. When I then added clip on ferrites it sounded worse - and I was not expecting that. I am not sure of the reason exactly - I suspect a resonance between the internal M scaler ferrites and the clip on ferrites air gap, making the loop impedance lower overall. But solid core ferrites do still improve the SQ - but I should add that it's not a huge difference. When I travel, I don't bother, as it's simply too small a difference and I stick to stock. Compared to the SQ that the M scaler adds, we are talking about the surface finish of the icing on top of the cake - it's the iced cake that's important.
But remember it's the overall loop impedance that's important, and the PSUs come into this too. If you replace the supplied PSU with linears, apart from breaking the warranty, will degrade the sound quality. This is because linear PSUs have no RF filters - the supplied one comes with input and output RF filters. Also the supplied one has been very carefully selected for no grounding, and very low inter-winding capacitance - many times smaller than toroidal transformers. The lower inter-winding capacitance, together with the RF filters, increase the loop impedance and improve the sound quality. By using a linear will increase noise floor modulation (making it sound brighter, fooling the audiophile into thinking it's more transparent) and increase correlated noise thereby degrading depth (and then the hapless audiophile will then convince themselves into thinking it's better because perceived width is artificially wider due to the poorer depth). Confirmation bias and placebo is a very real problem with listening tests!
But if you want to almost completely isolate the M scaler from the DAC then use a battery power bank (so long as you site it correctly) - then the loop impedance becomes very high, and then the BNC cables no longer have an impact on SQ as no current can now flow as there is no longer a loop. Is this something I do in practice - no - the difference isn't large enough to bother with charging batteries. At the end of the day I just want to simply enjoy my music more, and I don't need to go to silly lengths to do that.
, I've experienced many high end DACs over the years, I just prefer Chord but thanks 
