Help! Receiver or preamp? Im overwhelmed...

Hello jsbranham7!  I'm in just your situation, HT and surround sound music, hooray! I recommend the Emotiva MC-700 preamp/control center. It offers first class sound, but fewer decoding choices than a surround sound receiver. A used near top or top of the line Integra receiver can be had for less than $300 (likely $200 or less) and offers ten or more decoding choices (lots of fun) plus radio and the sound is B to B+ (quite good). You can always put it in another room if you upgrade to one of the very pricy Pre-Pros.  Use the "preamp outs" on the Integra receiver. I have three such setups running in my home. The Emotiva MC-700 controls the main system. There are five systems in all, one using the receiver as its maker intended.

As sort of mentioned, I would post this question on a HT forum, not here...this is predominantly a 2 channel forum, although some here still introduce a sub to a 2 channel set up, ugh...🙄
If speakers are not doing it for you in the bass dept, time for a better "pair" of speakers.

Hello, I haven’t experience to advise you, but I have read one interesting article about Preamp vs Receiver – Detailed Comparison.

Marantz is the best sound I’ve ever used.I bought them on eBay a year ago and they are still in perfect condition.

Seems like you like the Rotel sound. With music your main priority, Why spend the money on a video upgrade which may introduce a possible audio downgrade? You can get a 4K 4x1 HDMI switcher for about $300. It will have a 5.1 digital output to connect to the Rotel. Use rest of budget for bi wire or something.
You might find it interesting to disconnect your rears for a bit and use those channels to see how good the studio 100 would sound bi amped