RJR,
I have Klipsch RF-7's which are approximately the same physical size as your Zu's. You mentioned subs and I can tell you from experience a subwoofer will not solve your problem. By far the majority of audible bass in music is above the frequency range that subs are designed to accurately reproduce.
I would be a little concerned about the carpeted floor since your Zu's are vented on the bottom. I seem to recall the Decware radial speakers were vented on the bottom and in later versions they went to a fixed wood plate on the bottom. This provided more consistent results whether they were placed on a hard surface or carpet (the bottom gap and reflection surface remained constant regardless of floor surface).
You might try placing each speaker on a 16" x 16" 3/4" wood plank. I've even used bamboo cutting boards in the past. If you can find something your wife does not object to these have a second advantage. A plank makes it relatively easy to slide the speaker on carpet. This way when your wife is not around it is fairly easy to experiment with different speaker placements. Just place the speaker with spike feet on top of the plank.
In any event I would think the planks would reflect maximum low frequency energy out into the room. Most people think bass is 80 Hz and below. There is a lot of bass information going on all the way up to 260 Hz and you don't want to loose any of that to your plush well padded carpet.
http://www.independentrecording.net/irn/resources/freqchart/main_display.htmFrom everything I have read you have a fine amp and speakers. I am not familiar with your source ?? Are you confident it is cutting the mustard on the low end ?? Just ask because I recently replaced my 12 year old Zen Select with the Super Zen and was stunned how much more low end resolution the Super had. So obviously my amp was my weak link.
You mentioned auditioning equipment. I've found listening to a component in other peoples systems in their rooms is quite different than the same component in my system and my room. I'm sure with experimentation and patience you will work out the kinks with what you have.
Were you at last Fall's Zen Fest ?? Met a guy and his wife there that were considering Superfly's, they were from Champaign, IL. I live just south of Springfield, IL.