According to THX room size categories (copied/pasted from their website):
■ Compact Systems – small rooms, up to 1,000 cu/ft and a viewing distance of 8-10 feet
■ THX Select2 – medium home theater, up to 2,000 cu/ft and a viewing distance of 10-12 feet
■ THX Ultra2 – larger home theaters of 3,000 cu/ft in size and a viewing distance of 12 feet or greater
and
THX Reference Level: Designed to recreate the movie industry SPL setting, 85dB with 20dB of headroom. By turning the volume dial to “0” on a THX Certified receiver, users experience the exact volume level used by the sound designer – with the same level of sound fidelity.
So if you room is in fact medium size by THX classification, and your hearing is normal, at volume 0 you should be getting SPL similar to what you would experience in a THX certified cinema.
If you add an external amp such as the XPA-3, after proper "calibration", you will likely still have to set your volume to 0, hardly comforting visually speaking.
One last point, looking at the specs of your speakers, I don't think they are designed to play loud. The 1050's or 1050+ a 200WPC amp is actually a decent match for them, just don't expect by adding a 200W amp your will suddenly see the volume display lower.