att: kythynJust keep everything you want plugged into the "powered up" GTP-500 and use your source selector switch to select different sources, and the tape outputs of the 500, go to one of the inputs of the SYS, then the SYS becomes your volume control. This way your only using the 500 as a source selector.
This way you knock out the volume control and output gain stage of the GTP-500 from being used.
And you still have one input left on the SYS for your still another source.
Naturally the output of the SYS goes to your 555 power amp
Cheers George