The problem with a Y-splitter is that both signals have the full frequency range -- lows still go to the main speakers. (I know most subs have their own crossover to blocks highs.) I'm not familiar with how many of the class D amps you speak of have a high-pass filter for the mains, but I suspect not many, though I could easily be wrong. I have an Aiyima A70 and it's sub output does not have a crossover for the main outputs. I really like the way the Ultra coordinates the high and low pass filters for both sub and mains.