I have been using my GT-10 for a long time now and still don't have it all down, but it isn't working with my current amp. (my amps a piece of junk in the first place and doesn't have a effects loop)

So with that being said

I need some sort of speaker for my GT-10

Even though I haven't found the best way to dial in the tone, I don't mind the amp models built in and was reading about the Tech21 Power Engine as a speaker for it, but it isn't stereo which would be nice.

What amps would be best for me?

Would getting certain speaker work better with the GT-10
so you want a stereo amp right?
I wondered why the frisbee was getting bigger, then it hit me.
i've not tried that tech 21 amp you referred to. but i suspect it's an amp as well as a speaker as you put it. and generally speaking, tech21 stuff is of good quality imo.

at gig's with a PA you could run the GT10 direct to the PA or to an amp and mic'd to the PA.

as far as an amp goes a simple keyboard amp would work, either plugging into the front or into just the loop's return. the reason i say that is you are wanting your gt10 to work as the preamp by using it's models and the keyboard amp is going to have full range dynamics to it. (bass amps work like this too, but not nearly as well.)

a guitar amp like the tech 21 trademark 60 would work well too. and has decent pre-amp circuits you may like as well and you could use the gt10 in the loop.

dunno, just throwing some thoughts out there.

or you could grab a hybrid amp with a tube power section and plug the gt10 into the loop's return for some t00by goodness power amp tone.

basically a cofusing reply and bump all in one since i don't really know a budget here.
I wondered why the frisbee was getting bigger, then it hit me.