Tube amps usually put out the same power into 4 or 8 ohms so that might not make any difference. But I am surprised that your tube amp has to work so hard to drive your 90db sensitivity speakers. I have a much lower-powered tube amp driving Vandersteens (sensitivity 86 db) in a moderately large space to volumes higher than I want to listen to. Perhaps the input level to your amp is too low?
Sensitivity 86 v 90
I am considering an upgrade from speakers with a sensitivity rating of 90 db, 4 ohms, to larger speakers rated at 86db, 8 ohms nominal. Same brand, PMC. My tube integrated amp is 80-112 watts triode/ultralinear, and it’s fine for my 90 db speakers. Although, it is sometimes at around 4 o’clock on the volume control, approaching the max at 6. I am aware of the “amp power must double for each 3db increase in volume” rule of thumb, but really have no practical experience with this. I do like having 90 db efficiency, always assumed that meant a less powerful amp would suffice.
My question is, would the decreased efficiency be a concern?