Edmond - in that sized room I'd venture to say you might actually benefit from more power as you've got a big space to fill. On top of that, if you like to run it loud like you describe, then maybe a separate amp would improve your experience.
Like others have previously advised, I don't think you'd need anything more than a 2-channel amp to manage the front L+R speakers. The power requirements of the rest of the speakers are quite meager compared to those. (Perhaps with the exception of whatever you are running in Zone 2. I don't believe you've indicated what you're running there...)
To your question re: adapting XLR to RCA, check
these out as a $5 solution.
Mark makes a good point on fan noise. I've read quite a bit on these amps in the past (part of my addiction) and the general consensus is that as long as you get one of their newer lines with variable speed fans, fan noise is not reported as problematic. (just to reiterate, this is second hand knowledge based on what I've read). With the older amp models there is also apparently an easy way to modify the fans to make them run more quietly, but that sounds like too much work to me.
Jason
