Have you tried sustained the guitar to the GT-8 and then out to the amp?
I don't be versed how it would strong since you're essentially game 2 preamps that way.
I have a 75W Spider IV and they have no send or recurrence so that would seem to be your only opportunity without an AB box or something else.

I bought my Spider all in all for the 14-help coil participate.

Source: Guitar Forums: Ask The Pros: BOSS GT-8 & Line 6 spider 75W hook up???

by Andy

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Source: Boss GT-8 Guitar Effects Processor Multi Effects Board ...