What to upgrade next?

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.

Power cords and what you will get out of them hinges on your interconnects and speaker cables, the better those are the more difference power cords makes.

In what I'd consider a reference grade system, meaning the interconnects and speaker cables are super high resolution and neutral, power cords can have jaw dropping differences. The guys that say they can't hear a difference have serious interconnect and speaker cable deficiencies, they are the guys that tried many budget cables and believe they have found "the best" cables for their system.

Cables are loaded with catch-22's, great speaker cable will reveal nasties in weak interconnects you couldn't hear before, yet I think you start with a solid known quantity there, and then do your evaluation work on interconnects. If you don't the work you do comparing interconnects means nothing because there is so much you will not be hearing that you need to hear.

I start with a good neutral speaker cable, go to interconnects starting with the source to preamp, then the preamp to amp. To me those who aren't willing to spend some money on interconnects should get an excellent integrated. Once you have a high degree of resolution power cords will knock your socks off. Once you have good power cords you will hear differences that are staggering between good ones and great ones.

Cable has got to be the most contentious thing in the audiophile world. As someone who has not only sold cable to hundreds of people, I have tried complete arrays of dozens of cable lines. Good cable has little to do with price anymore, but you do have to spend some money to get really good interconnects, just not extreme money. Power cords are a lot of fun to try once the other cables are really good, Not so much before that. Also the differences in tubes are staggering once the i/c's, speaker cable, and power cords are there. The better the power cords the bigger the differences you'll hear in tubes. I like well matched cryo treated, read reviews on your given tube type to get an idea what the different characteristics of the tubes are. Tubes are last after the system's resolution is good enough to hear very subtle differences.

Wow Aintitgr8 that makes sense. Maybe that's why I can't hear any difference between the $35 and $750 I.C.'s. I also tried a PS Audio AC12 power cord and there was no difference. The speaker cables I have are Zu Mission with the BU3 connector on the speaker end for a good solid connection. Maybe an upgrade to Zu Event may help or yet another brand of speaker cables. It's so hard to really know what is the bottleneck in the system.