Zu Omen Defs and Ice Amps?

0.4 ohm is pretty high limiting DF to 20.

Here is an example of good inductor for the woofer:
http://www.parts-express.com/14-gauge-c-core-toroidal-inductors.cfm

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.

DF does not make much difference once it is higher than 100 since inductor in series with the woofer has typical 0.08 ohm making residual DF of 100. Changing amp's DF from 200 to 1000 will make only about 36% difference in overall DF (possibly less if you count cables connectors etc).

"of negative feedback switching power supply power amp."

It makes me suspicious of "negative placebo effect" when somebody has already opinion on switching in general.
Switching supplies are the same thing as class D amps. In addition to efficiency they have line and load regulation that linear power supplies are lacking. Latest $13k Rowland 625 - class A/B amp uses switching power supplies.

Noise from switching supplies is easier to clean because of higher frequency - that's why Rowland uses them in preamps where efficiency is of secondary importance.

Linear power supplies, if you thing about it, are really switchers operating at 120Hz poluting with 120Hz strong narrow current spikes of high harmonic content. This high harmonic content in addition to very high rms/average value ratio calls for oversized transformers. On the other hand ferrite transformer at 100kHz can carry 10x more power than toroidal 60Hz transformer of the same size. I would speculate that switching power supply, now days designed with zero voltage/zero current switching, got bad rap from extremely crude switching supplies in computers.

Why then linear supplies are still so popular? - for two reasons:
1. mistrust in new technologies (that was just expressed)creates demand for old technology (if it's heavy it has to be good).
2. any idiot can design linear supply - not so with the switcher (very complicated).

As for negative feedback - there is very little of it in class D since ouput is very linear by definition and output impedance is very low (inherent since output Mosfets always connect speaker to low impedances (GND and VCC). Iceamps have two feedbacks. One controls the timing of free running oscillator while the other is traditional analog one with perhaps only 20-30dB control.

So if anybody is objecting to class D technology or SMPS I'd like to add that delta-sigma converters, SACD and DSD operates on the same principle (Pulse Width Modulation)

Simon - some Rowland amps (like 301) based also on standard Icepower modules with built in SMPS use Power Factor Correction supply in addition that converts 120Hz charging spikes common to 120Hz rectification to smooth full cycle mains current. In addition PFC converts 110VAC to almost 400VDC making for much better operation of Icepower's SMPS(lower losses).

"Two of them" - My all thumbs to you but please promise that it will stay 2 (won't turn into 1).

Charles1dad - It is not being defensive but rather irritated with "All D amps suck" statements.