IMO it depends on your setup, goals, and the specific equipment in question.
- The turntable is the only thing you're amplifying and your amp has a built in phono preamp? You'll want the preamp between the two, just know that your EQ will go through the RIAA Curve in your phono preamplifier, so you'll be making small changes at say 100 Hz and bigger changes at say 1 kHz
- The turntable is the only thing you're amplifying and you have a dedicated phono preamplifier (or it's built into your record player). Put the EQ after the phono preamplifier
- You also want to amplify other audio sources. In this case I hope you have a dedicated phono preamp or your record player has line level outputs. You'll want to go record player to phono preamp to mixer (a receiver will work here) to the equalizer to the amplifier. Depending on the receiver in question, you might be able to do something like use tape monitor to feed the signal back into your receiver
If the EQ is scratchy, hit the pits with some kind of contact cleaner. Give them a quick lube once clean. This can be it's own rabbit hole, so I'll leave you to google/DDG/etc.