As for the Equalizer. Are the line levels between Amp and Pre-Amp not compatible with Equalizers?
Now that I think about it, I do seem to remember having a 12 band equalizer 30-40 years ago that was built from a kit by Synergistic Sounds, and when building it you had to make a choice between using it in the Tape loop or between the Pre-Amp and Amp. There were a few minor changes to some of the components [but I can't remember any of the details at this point, and the paperwork is out there in Never-Never Land if it even still exists]. But the advantage of having it between Pre-Amp and Amp was that you could tune your system to your specific environment and speakers with a "Pink Noise" source using the calibrated mic that came with the Sansui SE-9 Equalizer and its 8 band/channel real time display. And, of course, this would result in all your sources [tapes, vinyl, tuner] getting an almost neutral environment to play back in. The Synergistic Sounds Equalizer got torched in a house fire 9 years ago [mostly smoke damage and possibly some heat] but I still have the actual unit itself. I suppose it might be salvageable, tho I might have to change out the 24 slide pots as they where mostly plastic cased, and as I mentioned before, I haven't seen any of the documentation since the fire.