What to upgrade next?

Costing absolutely nothing but time and effort, experimenting with loudspeaker and listener positioning can produce very positive results.  If you want to spend money you could look at isolation/resonance suppressing devices such as racks and footers.  Finally have you thought about the Zu version of the Denon cartridge?

Of the three choices you listed to me absolutely power tubes. You can do much better than JJ and the change in sound quality is  dramatic and substantial with better tubes. That being said ohhwy61 has some good thoughts above on positioning which is indeed free!

Yes we are in the process of changing our generic entertainment center to a custom walnut slab unit. Not necessarily specifically made for audio but it is very solid and wifey approved. The TT came with 3 point isolation pointed feet so hopefully that is correct isolation. I thought about the Zu version of that cartridge but they were out of stock a while back when I was looking into it. I've since come across SoundSmith and was wondering how much improvement that brand might bring in. I can move the speakers around a little bit but the location in our room is a bit limited. 

Rick