The M and more so TBR series require a high technical acumen to dial in. Once you get it, they’re amazing, but you have to understand the controls and how they relate.
The Knucklehead was less tweak ability and updated for 90’s rock/metal.
These amps are VERY sensitive to tube swaps. I ran all Mullards in my M60 and the thing just breathes. Running the SL channel with both gains off yielded plexi tones that crushed for stuff like Thin Lizzy when the master is opened up. The Slavemaster gets op amp Big Muff tone with ALL the articulation you miss from the pedal.
At the end of the day they are boutique amps for masters, and that’s why they had so many top players in their artist roster.
With today’s degenerating level of intelligence, I fear the common player just can’t grasp the value of such works of art.