Help! Receiver or preamp? Im overwhelmed...

I have a Rotel prepro, and if you like the sound you’re getting now I wouldn’t recommend Marantz, which has a very different sound profile with a softer and less detailed treble but a fuller, richer midrange.  If you’d like to keep the basic sonic traits you have but just improve it across the board I’d go with Anthem.  There’s a used MRX720 on US AudioMart for $1600 (I’d offer $1300 as Anthem just introduced their new models) that would provide better 2-channel performance but also much better multichannel processing and room correction.  It’s a bit over your budget but a nice step up if you can swing it, but if not you can probably get a used MRX520 for around $1000 but I think is a step down in their ARC room correction.  Personally, if I’m you I’d buy the MRX720 as it’ll give you great all-around improvements and I think you’ll be thrilled.  Anyway, FWIW and best of luck. 

@noble100 

You say that an Oppo 205 would do the trick - but can it control volume with the analogue outputs? I ask this because my top end Panasonic 9000 can't - I needed to get a volume control with a remote - so I just caved and bought the best sounding processor I could find used. That was a Classe 800 - it sounds amazing, but can't do 4K - which is why it didn't cost $15K. Compromises, compromises ...

Denon X series, if you already have a five channel amp and want to use that in conjunction of the Denon’s preamp out it’ll work great. Just get an X3700 and be done with it. Best performing processor receiver for the money by far. 

Love my Pioneer SC LX81 but depressed looking at what they're worth now; sounds great with 2 ch and 5.1.

My LG is OLED but pre 4K and still does it for me- still a fantastic easy to watch tellie

The Anthem MX 520 cost me about $1100 when purchased from my dealer.  There may have been a slight price increase in the year since, but it should be close to enough to your budget to justify a little stretch and the advantage of owning a new component.  I greatly prefer Anthem sound without Room Correction to Marantz and Denon.  With ARC, it’s not even close, imo