I don't have the Scott 121-C schematic in front of me, but many pramp internal gain stages that I've seen can be bypassed by using the Tape Monitor Out jack.
Your cartridge's output would go into one of the magnetic inputs on the Scott, and then an adapter as shown above would be used between the Scott's Monitor jack and into a spare set of line inputs on your main preamp.
You're essentially using the Scott as a separate phono stage for RIAA (or other curve) equalization, and your line stage for preamplification.
