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you can have one device for capture (input; ie. your fender usb amp) and yet another for playback (output; eg. your pc-soundcard);
check that out on qjackctl/Setup/Settings/Input Device (-C), Output Device (-P) resp.
you must revert the Audio setting to "Duplex" though.
once you set any of those fields, the main Interface one is of no consequence.
oh. and possibly both devices must support and function at very same sample-rate (-r), period-size (-p), number of buffers (-n) and what not, but if you set them with some conservative figures i guess it will be alright (eg. -r48000 -p128 -n3)
hth.
byee