Minimizing Audio Feedback
Feedback is simply a function of how much your signal is getting amplified (gain) and how sensitive/close your audio source is to the output (probably your guitar pickups).
This means that by pure physics, having a high volume, high distortion (and especially over driven) sound, whilst using a guitar with sensitive/high output pickups, and having your speaker be very close to those pickups (like in attachment mode) is destined to cause feedback.
So between:
Attaching
High distortion/gain/overdrive
High volume
You kind of have to pick two.
That being said, there is a lot you can do with the Noise Gate first.
Increase the slider to the right on the Noise Gate Threshold such that it basically mutes the guitar when you’re not playing, but not too much more. This may take some experimenting. It’s also a good idea to lengthen the release time to make it sound a bit more natural.
So the Noise Gate will help allow you to maximize the volume and gain when in attachment mode.
Otherwise, you can detach and crank it all the way up.
I hope that’s helpful.