What to upgrade next?

What I am looking for is better separation and clarity and dynamics from the upgrade.
Yeah I forgot to mention the phono stage I am using is a PS Audio Nuwave. You just never know where the weak link is until you try something else. 
If i do decide to replace the tubes, I am pretty sure I will go with the KR300b HP. It's supposed to be their high performance tube made with top notch parts. 

I vote for changing out the jj 300b,to anything else including used 300b's that are in your price range.Does this Intergrated use a 6922 and a 5687,if they are jj's I would look at changing them as well.

I would go after a better pwr cord as well,lots of good ones that don't hurt the pocket too bad.

Offcourse all the tweaks already mentioned usually provide good results too.

Your cartridge is a good one,if you really play a lot of vinyl maybe look at a upgrade in the future.

Just my 2 cents,
Kenny.

Tubes. Zu Denon. 

I find OP's question very difficult to answer. Different tweaks tend to result in improvements in different areas in sound quality. Some add a little more bass - my experience with good power cords, or give wider detail- my experience with cartridge, etc., etc. It really depends what specific deficiency in sound you're trying to alleviate. As for tube rolling, I have no experience in that space but I'm sure the same basic principles apply.