I am trying to decide between these amps. I have an Onkyo tx-nr818 that is pushing polk rtia9's,csia6, fxia6. also have the mains bi-amped. obviously I am seriously under powering this setup. with the xpr-2 I could drive the mains with 600W each and the onkyo should adequately drive the center and rears. at $1600 is it that much of an improvement over getting the xpa-5 and just bi-amping the fronts to reach 400 watts, and using the remaining channel to drive the center? at that point I could use the $800 I saved from not getting the xpr-2 and use it to get my 2nd PSA XV15. a 3rd possibility is getting the xpa-3 and that could give 200W to each of the mains and the center. is it just headroom that I am gaining from each step up in the emotive line or is there a major difference in overall sound?