Ok, here’s what I’d do. @fuzztone is right that the preamp section of the Denon will compromise your stereo performance so adding an amp will only get you so far. Because this is mainly an HT setup, rather than getting into separates I’d go for a better AVR like one from Anthem that’ll get you both better stereo and HT performance. If you don’t need the latest bells & whistles you can get a previous generation Anthem AVR at a relative bargain price and sink the rest into a better center speaker (which is critical IME) and a decent TT. Here’s an MRX720 that’d probably work great for your situation...
https://www.usaudiomart.com/details/649773664-anthem-mrx-720/
For a center channel speaker I think a good used B&W (one with the tweeter located above the mid) would work well — something like this...
https://www.usaudiomart.com/details/649742814-bampw-htm2-center-speaker-black/
Just a couple ideas FWIW, and best of luck.
https://www.usaudiomart.com/details/649773664-anthem-mrx-720/
For a center channel speaker I think a good used B&W (one with the tweeter located above the mid) would work well — something like this...
https://www.usaudiomart.com/details/649742814-bampw-htm2-center-speaker-black/
Just a couple ideas FWIW, and best of luck.