I'll second Jeffloistarca regarding stands. No matter which of these terrific monitors you choose, you'll absolutely need a pair of high-quality stands factored into the equation.
I posted a thread a few weeks ago which was very similar. I just moved into a 10' X 12' listening room and was looking for a compatible pair of speakers.
I was hoping that I might be able to get by with a smaller pair of Maggies, like the MG12's. With this small of a room, it would have had some significant trade-offs, so I decided to concentrate on a small monitor that can be placed relatively close to the wall.
Some of the recommendations included the following:
Totem Model 1
Totem Arro
ProAc 1SC
Spica TC-50
ACI Sapphire (I'd love to hear these)
Audio Physic Step
I ended up with a pair of Aerial Acoustics Model 5's. I just got them about a week ago, so I'm still working on finding the optimal placement (plus I'm waiting for a pair of Osiris stands that I ordered from Avalon Audio Video - $299 shipped!!), but they work EXTREMELY well and are VERY musical. Huge soundstage, detail without being bright or forward, and they don't seem to be too finicky about placement.
Your system allows you to have a LOT of flexibility in selecting a speaker - lots of power and current - nice warm tube preamp. I haven't heard the ACI's or the Piega's. Between the remainder on your list, I would have to give the nod (by a good margin) to the Totem Model 1, followed by the Silverline then the ProAc. I've never been a big fan of Monitor Audio speakers - not sure what it is, but they just don't sound very musical to these ears.
The good news is that you've got a lot of great choices. Have some fun and don't be swayed by others pushing their opinions. You're the one who will ultimately be living with the system. If a speaker sounds good to you (in YOUR room with YOUR equipment), that's all that matters.