I've owned the M-150s for over 10 years now. I've found tube rolling can get great results with these amps. 12ax7s, 12au7s, EI34s, Kt88s, all can make a difference.The 2 12au7s will make the least different. Mullard NOS and other 12ax7s can really tone down "brightness. I occasionally use NOS Mullard CV491s as the input tube in each amp. They are equivalent to 12AU7s. I lose a bit of gain but they sound great. Usually I use Valvos as the input tube in each amp.
I have a bunch of gear. The M-150s used now are a great buy for the money.