Upgraded turntable - is my current setup sufficient?

just to confirm, your denon is vintage dp 3000? or a typo, is it the current dp300?

either one, you are changing from direct drive to belt drive, I prefer DD, quartz locked (on, instant and maintained speed); but if you didn't/don't hear any problem, don't have power variations, after motor warm up, speed of belt drive should be stable.

IOW, I don't think the table itself will make a difference, and I think other things done correctly, more expensive tables will also not be as important as the arm, cartridge, phono stage. 

If vintage dp 3000, you didn't mention current arm. 

cartridge, the new clearaudio will definitely sound different, and should also be an obvious improvement.

However, it is an elliptical stylus shape, so even if better, a future upgrade to advanced stylus shape, Microlinear, Fineline, SAS will be a definitely audible further step up.

It has much tighter channel balance .4db existing is 1.5db. that will definitely tighten the center resulting in improved imaging. channel separation specs are very similar 25/26 db.

Signal strength: new clearaudio is lower 3.6mV signal; existing is 5.0 mv. You will definitely need more juice out of your preamp/amp for same volume as existing cartridge. If it remains quiet, no concern. If tubes, you will be driving them harder. Great results, who cares. 

Cartridge change n the future, I would consider higher signal strength, lighter tracking force, wider separation, keep tight channel balance.

I would assume your turntable is dp300f.
The Marantz turntable dwarfs rest of your components. I think the biggest bottleneck is the phono pre. Then, your speakers, and preamp, and then amp.
Your turntable and cartridge deserve at least $300 phono preamp. Your Rotel amp and Adcom preamp will be OK for a while, but you would not get 100% out of the turntable.
My suggestion is, seriously looking for better phono pre ASAP, and then save $$$$ for next speakers and amps.

Yes - dp300f. No typos - also it is belt driven. Stock arm and upgraded to sumiko rainier cart. 

Any suggestions on a phono-pre that pairs well with the clearaudio cart? 

I don't see myself upgrading the rotel amps or speakers for at least a couple years... Though you never know should a good deal come around. 

Experimenting with some different phono's seems like an easy enough exercise though. Thank you. 

The Schiit Mani has got to out-perforn the Adcom. I'd start there. You can also add a linear power supply for an uptick in SQ.

I think your question was whether you would be able to appreciate an improvement from your new Marantz rig, in the context of your present system, compared to the denon that you now own. My answer is a tentative yes. Beyond that I would take with a grain of salt the tons of conflicting advice that your post has elicited and will continue to elicit. In the end you have to make up your own mind. I would say that the very last thing I would do now if I were you is to tie up any money in a synergistic PHT thingy. Millercarbon seems to love them, but there are many among us who think they are totally worthless. It’s debatable. However, whether worthless or miraculous, making a tweak like that a priority is inappropriate at this stage.