Tube or monos for ProAc Response 48D

Having direct experience with pretty much the entire history of Emotiva amps, I would avoid the newest "Gen 3" amps (such as the XPA-DR1 you were considering). Your older XPA-2 is going to have much more bass power and will sound more natural than the new model. The problem with the new model is Emotiva’s decision to go to a switching power supply, which reduced the shipping weight enormously, but took a nose dive in overall sound quality (in my opinion). I have the XPA-DR2 in my own system and it just does not sound as good (even when highly modified and rebuilt). If you wanted a bump in performance, try upgrading the XPA-2 with better fuses (such as Furutech if you can find them, or something like BLUE. Isoclean is a possibilty too).  I went back to the original XPR-1 monoblocks, which just kick butt over the new amps and many others.