I've run power soaks, and other resistor-style attenuators on my mark2b for years now.
when i used to gig full time, i had one of the original scholz power soaks on my boogie, just to tame a bit of stage volume, and never ever had a problem.
the tubes only wear out as fast as they would if you weren't running the attenuator, and at the louder volume.
i can tell you from first hand experience, i've never had a problem.
run a fan on the tubes, to increase the life.
in fact, i just purchased a WEBER mass lite, to run for recording purposes, and purchased a Palmer PDI-09 (which i've already used, with great sucess) and plan on running the weber after the palmer, just for volume control for monitoring purposes, as the only signal that will get recorded is the palmer signal.
maybe i'll try mixing some live cab sound (miced) with the palmer,
i'll follow up with a critique of that soon.