Help! Receiver or preamp? Im overwhelmed...

What's your budget?   This will probably dictate what we recommend.  Such as under $1000.  $1000-3000.  $4,000 or more. lol.

What’s your budget?

The OP asked for help on the receiver/preamp side of things, so I don’t think we should be overloading the response ledger with disc player recommendations.  So any, a few observations.

1) Budget is important, as others have noted
2) Latest HDMI?  First of all, that is a moving target, as all of us have to deal with.  It changes frequently.  Otoh, since you are willing to buy used, that can be an advantage.  Many a consumer has moved on from a perfectly good component because the HDMI standard has changed.  If your monitor, disc player, and other sources are happy with an older standard, then you will have quite the market to choose from.
3) Consider Room Correction.  IMO, this is the biggest sonic advance in HT over the past 20 years, particularly if music is the main priority.  For me this narrows the choice down to three companies: Anthem, Yamaha, and NAD.  Anthem and Yamaha use their own proprietary RC, and NAD uses a third party, Dirac.  I haven’t heard a NAD, but I use Dirac from another vendor in my 2 channel system and love it.  I have 2 Anthem 5.1 AVRs (MX 520) both were huge upgrades over the Onkyo and Pioneer ES that preceded them.  I heard the Yamaha demoed at my dealer and it wasn’t as immersive as Anthem but it is very good with dialogue.  Again, your priority is music, so I’d go with Anthem