I had both heads

I prefer TM18: both feature-wise and tone-wise.
Not that GM15 is a bad amp, but especially the gain tone was a little bit off for me.
There was some component in there (in the tone) which got on my nerves.
It could be the sum of all parts: cabinet, room and whatnot of course.
TM18 however delivered very nice tones: clean, break up and hi-gain too
(but not modern metal hi-gain type of sound).
I only returned it because I needed money for a other amp I really wanted (it got very good deall on it).
BTW at the Musimmesse there was new Tubemeister 36 amp presented
I have tested it and it's great: 3 channels with 2 EQ sections and MIDI switching including Powersoak setting!
I don't know the price, but if it's not too high, it will be a killer