I feel with the latest DAPs the SQ difference will be very subtle and only by nitpicking will you get any difference. Perhaps it's time to turn my attention to headphone mods where the difference will no doubt be substantial either positive or negative.
That's actually my next step, I already reached the maximum I can get through my phones, mods for them isn't gonna bring much so I am going to have to swallow the incoming costs of new pairs. You are right though the DX90 as such is a very good player with its current architecture and setup so to squeeze a few drops of SQ we'll have to move to the next audiophile step, that means better phones, more money, a thinner wallet and hopefully a grinner smile on one's face when achieved.
The slow roll-off has been so much talked about on the DX50 thread and I know that it would have improved the soundstage, in fact I though the 1.2.3beta was released with the slow roll-off on, just to get it removed when rolling the final update but no options to select it like this upgrade.
This is what ESS advertised on their DAC literature:
OVERSAMPLING FILTER: Fast/Slow roll-off (PCM), it's very similar to the features provided by the WS8740 DAC.