Yes, just hook up the table to the GTP-500's phono input and use the preamp outputs to any line level input on your GTP-600. This will allow you to use the volume control on the GTP-500 to match levels, however a cleaner signal would be to use the tape outputs from the GTP-500 to any line level input on the GTP-600. If you do it this way, you will have a fixed output, but a cleaner signal path.
Can I connect 2 Adcom Preamps?
I am new to this and I have the following system, Adcom GTP-600 preamp, Adcom GFA-5500 and Adcom GFA-6000 amps. I have been running my phono through a Recoton converter. Recently I acquired an Adcom GTP-500 preamp which has phono inputs, but not enough space for everything I have. Is it possible to just add this preamp to the setup I already have?