Input to Use for a Phono stage

Any line level input will work. There's no preference between them, as they all function the same. You don't say if your amp has a Phono input, but if it does, do NOT plug the Slee into it — the result would be gross distortion.
You are now using MP, CD, AUX. I'd suggest Tape 2, using the "Play" inputs on the back. Tape Monitor may have an extra button to push, which can get confusing. Just select your Tape 2 when playing vinyl, and be sure Monitor is switched off.
I used a Tape input for my phono preamp for weeks, it sounded fine.

Try all line-level inputs -see if any one sounds the best.

All of the line level inputs go to the same place. Or follow bimasta’s suggestion, if you don’t want to disconnect the gear already connected to the line level inputs, in favor of your new phono stage.