I used the U70 with a pair of Ohm 2000's and it was plenty loud enough without any noticeable strain. IMO the room size is an essential parameter, as perceived loudness is mostly a three-parameter issue (assuming standard linestage levels): speaker efficiency/sensitivity, amp wattage, and room volume. You can sit closer to your speakers and hear them louder, so psychoacoustics enters into it as well. To see if my setup is "loud enough", I go as far away as possible from the speakers and blast them a bit louder than I can stand it long term--the U70 always passed that test in the 2000 ft^3 room that I used it in and I am pretty sure that it would have passed that test in my current 5000 ft^3 room with 15" Tannoys.
The loudness-killer combo is a large room and 80something db efficient speakers that cannot take 1000's of watts (i.e. a very large percentage of "audiophile" speakers nowadays). An excellent 35-watt tube amp should be fine for daily sane usage in just about all settings, and if it isn't then the first place to look is probably at the speakers, then the amp (assuming that the room is the fixed variable).
If you want it to be as loud as it is at your favorite nightclub, then you might need to get a pair of pro speakers (say, some JBLs) which hover around 100db for 1 watt, and in that case the UltraValve would take you up to 115db or even a bit higher, and that is pretty darned loud.
For comparison, sitting in what I think are the best seats at the Disney concert hall here in L.A. (not too close, not too far), I heard a performance of Mahler's 6th and in the final movement when the big hammer blows come down, the peaks were right around 100db (as per radioshack spl meter). If I'd sat closer then it would have been louder but frankly I like it better not quite so loud!