Hi,
I've owned the Pass XA 100.5 mono amps for a little over a year, but recently sold them. I've also owned H2O Audio class D amps for a long time, and am currently using H2O mono amps to drive my Avalon speakers. So, I have compared both these amps directly with my speakers and have also listened to both amps extensively with the Magico V2 speakers. The verdict? They are both great when set up within the right system.
I think both the Pass and the H2O amps sound wonderful but in different ways, and my preference comes down to system matching. The Pass XA.5 amps have a full, warm sound with a glorious midrange and very smooth high frequencies, and sounded beautiful with the Avalons. However, the Pass amps don't have the control in the low end that class D amps generally exhibit.
The H2O mono amps don't flesh out the midrange as much as the Pass amps do, but they are still wonderfully liquid sounding through the midrange. They can be amazingly good sounding in the right system. I preferred them to the Pass when mated with the Magico V2.
Alan