Improving existing setup, where to start?

Baby steps...

Replace Red stylus with Blue stylus ($160 if you shop around and you then have a backup/Red stylus - a backup is always good to have on hand).

Listen for a few hundred hours, and if not satisfied look into a different outboard phono preamp.

Also, check/redo cartridge alignment/setup if you are capable of such.

If not, then learn how to DIY.

DeKay

I’m not familiar with your amplifier but one of the Rogue tube integrated amps has a built in phono stage. I’m somewhat skeptical about replacing the Marantz based on your budget. My understanding is that phono amps really begin to stand out at around the $2,300.00 mark. So if it were me and I had to be fiscally conservative, I’d first buy the best cartridge I could afford.
As far as nice stand alone phono amps, there are plenty on the used market. For whatever reason, good tube phono amps tend to be a good deal more expensive than solid state. I have an ASR Mini Basis Exclusive and I believe it’s very good for the money and it offers the tone of a tube amp with the bass and transients of a solid state. I need a better tonearm and then a better mono cartridge and I’ll have for the most part completed my analog setup.
If you’re satisfied with the sound after you upgrade the cartridge, then relax. It’s all about the music. I’m saying this as I’m spinning a 1954 Haydn Society box set of vinyl remastered from 1938 recordings.

Thank you guys for the replies.

DeKay, I'm almost certain now that this is a great starting point. Seems very easy to swap the Blue stylus out and then ensure the cart is perfectly setup. 

The damn thing came with a Goldring Eroica high output moving coil and I was using a Jolida tube driven preamp, sounded amazing and I've chased that sound ever since. I think one of my kids messed with the table and broke the stylus. At the time, I had no idea I could just replace the stylus so I just threw a cheap cart on it and stopped using it. 

Anyway, Blue stylus and proper setup followed by phono amp. Perfect, thanks!

Yes mani schiit is  very very good, I like the speed and the explosiveness of this phono preamp, it match well will ortofon blue mm...