Tube amp, size and power info........

I am geting a new pair of speakers wich are 99db efficient with a plate amp for the woofer, I have a tube pre but now I can entertain tube power but I want a reasonable price to try and also it needs to fit in a standard rack space (6.5 inches) I would say that anything above 25 watts will do, my pre-amp is a Sonic Frontiers and my usual volume is low to moderate, never obnoxiuosly loud, what are some options that wont break the bank, I know it isnt much but I am talking around $500 new or used.
Thank you as always Chad