My zen in office (8x10 room) on 89db Trapeziums... plenty loud.
Same zen at home in a big open area (living, family, kitchen, etc) on 96db HDT's.... not quite enough for me. So I got a Torii.

Traps are 4 ohm while CM5 are 8 ohm.   Zen (assuming default transformers) will have a bit more power on lower impedance speakers.

when I play loud, I set the knob at noon (mid) or even 3'clock depending on the song.  I'm guessing with less efficient speakers and less power transfer into 8 ohm, you'll use the 2nd half of the volume more than me for a 8x10 room.

for low, don't bother the people in the next room, listening, you'll be fine in a small room.

So I'm listening to the Zen amp right now with my B&W speakers. Had it running for about 2 hours now, listening to different albums and genres.

It goes surprisingly loud. I have the volume turned up to around the 2:00 position and it's basically at what I call a moderate listening volume. I can turn it up so that it goes quite loud, with no sign of clipping at all.

Very neat little amp!

The most outstanding thing about the sound is the expansiveness of it. The imaging and soundstage is wide and deep.

There's a fair bit less bass than there was using my solid state amp, though. This could be mainly due to the relative inefficiency of my speakers.

Just to answer my own post here, I tried out the Zen amp with a pair of Klipsch speakers (RF-600b). I know these aren't what we consider "audiophile" speakers, but at 97db, these things really let the amp show what it is really capable of.

I actually don't mind Klipsch speakers, if you pair them with the right amp. These certainly don't hold any high-end snob appeal, or anything like that. The home theatre crowd would probably be wowed by them, for sure.

Gonna spend some more time with them today paired up with the Zen.

I could play the little Zen amp to absolutely insane volume levels with the Klipsch. Only problem is, the Klipsch speakers have no mid-range whatsoever. Lots of clear highs, and crazy bass. But no mids.