Yeah, apart from personal preferences (I personally think the M III is an awesome amp and incredible value for money), when you plug into the return you bypass the whole modeling circuit and get to a great sounding solid state power amp.
Also I have found the Celestion G12T-100 better sounding than the seventy 80 (which I think is a little dull). The G12T doesn't sound dark.
The Seventy80 does sound very bare bones, no character of its own.
But when A/Bing it against my Dynaudio BM5As (I was always a nut about the idea of taking my studio tone on stage from way back when), it is almost apples to apples; which is why I highly recommend the Tech 21 for use with modellers. What you put into it is what you get out of it.
Of course, thinking about it, perhaps it is a bit dull! Guitar speakers like the Celestion I had in my old Marshall (still in the family, with my brother :P) give the sound more bite and have an edgy/sparkly quality, especially when used with distortion.
One workaround to the open back design of the Power Engine is the EQ controls, something you would miss when going in through the FX return of the Mustang. These can be used to tune the Tech 21 to be used in a variety of environments. I think the open back design gives a more mid character to the amp, whereas a closed cabinet has more pronounced bass frequencies. This makes it harder to go muddy, but could also confuse you thoroughly
I really wish I could A/B with a Mustang to figure out whether I should upgrade. Then again, I think my GAS should remain limited to the Kemper for the time being