What to upgrade next?

If you wanted to make an upgrade which would be the next most noticeable improvement. 

1: Power Cord (currently just stock cord)
2: Power tubes (currently JJ's 2 each 300b)
3: Cartridge (currently running Denon 103R)

My system consists of Audion Silver Night Special Edition 300b integrated, Acoustic Solid TT, Zu Soul Superfly speakers with Mission cables. 

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. 
You already have a very nice system so I am surprised that no one has mentioned room treatments?

Before upgrading anything else, treat the reflection points in your listening space. Put some bass traps in the corners. There are some very simple DIY projects that don’t cost a lot of $$. You want to hang things that can absorb reflections.

I initially stuffed some large bean bag chairs into the corners behind my speakers to see if I could hear a difference (I could). But any large stuffed pillow could work.

As a test, you could also try hanging heavy velour curtains at reflection points to see what kind of effect (sonically) they have.

And play with speaker positioning.