Audiofeil - I agree. I use small class D Rowland 102 amp with Hyperion HPS-938 speakers (90dB sensitivity). Sound has presence and resolution but also softness and fullness. Bass is punchy and powerful. Instruments sound very natural with great harmonic structure. There are even many better class D amps these days, so it is hard to understand how anybody can toss general statements like that.
Class D amps are very revealing, I'm sure you agree, ruthlessly unmasking any system deficiencies - otherwise it would be hard to understand why others describe similar amp as "ear bleeding", "dry" "thin" etc. while I hear just the opposite - smooth, clean and full sounding.
"The Zu's obviously don't need a lot of power so I'm thinking one of these newer digital amps might be a good fit."
Texas42 - Digital amps are "a lot of power". I often see such statements relating to lower power tube amps and high sensitivity speakers. More power won't hurt but there is a risk of overdamping, as Paul suggested - need to check synergy. It worked for me in spite of 90dB rating.