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.