Prior to my purchase, I A/B'd the GM40D with a TM20D and a TM36.
For my needs, the GM40D excels at versatility, as long has on is used to H&K's trademark "hi-fi" sound.
For usabilty, the amp has 4 channels going from cleanest of cleans to highest of high gain. It does give a Recto or a 6105+ a good fight, for what Metal players are concerned.
The gain levels are consecutive between channels, meaning highest gain on the Clean channel is followed by the lowest gain level on the Crunch channel. The EQ is also more responsive than on the TMs, and the amp isn't trebley as the TMs are. The voicing on all four channels is quite warm, as I felt the voicing on the clean and crunch channels on the TMs to be colder. Of course, everyone's ears are different, but on the GM40D I can go from Fender clean to Marshall Crunch to Marshall High Gain to Mesa/Peavey brootalz, and all type of gain and sound in-between.
Than there's the control factor: everything on the GM40D, apart from the Master Volume, is MIDI controlable. For someone trying to keep a small amount of gear and corresponding real estate, a GMD40D and a MIDI board is all you need to play live.
On top of all this, is time for some ear candy: The amp can also have Boost on/off on each channel, and the voicing on the boost is different on each channel. To keep things quiet, there's a noise gate, it's threshold controlled via a pot on the back of the amp, and, to keep the real estate really small, really good built in effects: Modulation (Chorus, Tremolo, Phaser and Flanger, one at a time), Delay and Reverb. Not really pristine or vintage effects, but all are really usable (As a matter of fact, I'm only using a wah as an outboard effect).
If other effects are available, there's also an effects loop.
As extra but useful features, there are 4 power levels available (which of course affect the behaviour of the amp accordingly): 40W, 20W, 5W and 1W. Silent recording is also possible if using the Red Box Out (DI box with speaker emulation), which also works really well if plugged to an interface or mixer for silent practice with headphones.
Considering all, and even if more expensive than any of the TMs, both the features and the quality of sound made this amp a real winner for me.