I recently did something similar to what it sounds like you are trying to accomplish so I will tell you essentially everything I know about this stuff (which as you'll see is very little) and hope some of it is helpful.
If you found outdoor speakers that you like, stick with them. After listening to a lot of speakers (Polk, Klipsch, Infinity, Bose, - everything at Best Buy and Circuit City) and really wanting to like Polk or Klipsch - the hands down winner (meaning in this case it was the "least bad" option) was a $40 pair of speakers made by Dual.
If you are going to run multiple zones off of a single output you need a hearty receiver or amplifier. My Yamaha HTR 5640 couldn't handle it (3 zones through a Russound SDB 4.1 "speaker selector http://www.russound.com/speaker_selectors.htm
- but it was designed for home theater, not stereo with really low resistance. HK stereo (3480) and AVR (235) handle it very well although the truth is that a robust stereo amp would likely be a significant upgrade. The more zones you are going to try to run with one amp source, the more this will be true.
I have volume controls similar to the ones you linked. They do not seem to degrade the sound in any way and having the ability to put on 5 CDs, hit "play" and then adjust the volume in the individual locations is very nice.
Given your space limitation, I'd be willing to bet that you will have a hard time finding anything that will compare favorably to "free" Polks. I have a Velodyne CHT -8 subwoofer in a cabinet (yes, I understand the cost and cosmesis limitations of what you are going through) in my living room and when I crank the music in there, the bass can be appreciated in the other 2 zones where there is no sub and the speakers are less than ideal... so you might want to put a decent, small, very inexpensive sub (I got mine for $150) in the room where you are most likely to play the music loud and let it "supplement" the other zones.
Hope this helps.
