OD in the loop?

No, it won't damage the amp. The voltage levels that come out of the GT8 won't be anywhere near enough to damage the amp. It's more likely to damage your ears if you're not expecting it get loud really quick when you change to a loud patch.

 

Safety wise, its fine in either case explained below

An OD "pedal" in the loop from a technical perspective is fine when concerned with safely running the amp. The FXSend/Pre-Amp out is simply a line level hi impedance unbalanced signal. But since its post Pre-amp and fx loops typically have some pre buffering or even added gain either pre or post, its not the best place for an OD "pedal"

But you using the GT-8, a guitar modelling processor which is more loop friendly since it uses input and output buffering which will play nicer with the fx loop. Its a line level compatible device more or less.

An OD pedal is really suited to "seeing" the low level high impedance signal from guitar pickups and its true character is designed to come through this way. Its not that it doesn't work in a loop, it just does not work as intentioned. Factor in that some Dist/Fuzz pedals have a higher than normal impedance to increase their input sensitivity and you could be throwing away some headroom.

But with the GT-8, all bets are off in this regard since its a modeller and is line level device friendly. You could probably use this thing into a home stereo. I went digital way back in 1990 and ultimately realized its not for me but it was useful for direct in to a console channel, power amp or even stereo.

Is it wrong to put it in the loop, no, just unconventional but if you like the sound of it, to hell with convention.

But I am going to be honest, your tone sucking issue is not the difference of the GT-8 on the floor or in the loop, its the GT-8 itself. Its converting (digitizing) your signal and although this is a contentious topic (digital versus analog) I think it is generally safe to say that analog is better at rendering the same as you put in versus digital where the object is to model or sample, meaning a representation of your original tone.

Concern for tone alteration/sucking with digital gear is par for the course.

This may not be "as true" for general audio, home theatre/hifi, recording or sound reinforcement, areas where digital excells but for the RC coupled electric guitar using magnetic pickups, I believe it is just that.

Bottom line, if you like the way the GT-8 sounds in the loop, carry on.

 

I will bet the FX sound great, the time domain fx in those things are always nice and in the loop even better.