Most significant upgrade

Yes, a toy. As I suggested, try Nottingham or one of those German tables - Acoustic Signature, Acoustic Solid etc. and you will agree. Yes, of course, it'll cost some money.
Well, at the very least you could upgrade to P7.
Speaking financially, serious analog starts at about $3000 new for table/arm. Plus cartridge.
You asked for significant upgarde, that's my opinion. You like your speakers so I said nothing about the possibility of upgrading them.

jake, thanks for the positive thoughts on tuning the room. I've been doing some extensive reading on the subject and have tried a few cheap diy versions with great success,,,,,,,great input brother, Thanks!

"Inna" bad mood? Funny, Some dudes that have been in the vinyl bizz for longer than I have been alive strongly recommend the P5 over the P7 as the increase in cost does not justify the return on performance, i.e. Michael Freemer and Art Dudely

http://www.stereophile.com/turntables/1204rega

I'm quite sure the Germans make a great table, hell, I work for a German company for Christ sake but to call something a toy? Come on dude, you can be more creative than that? Seriously I appreciate the opinion, just keep it peacefull man............It's just music for the soul.

More creative...I'll try. You don't seem to want to upgrade either front end or speakers. Or amp, or cables.
In a situation like that there is nothing that I could recommend. You could call my Audiolab integrated a toy too because it is though a pretty good one.

Jim, what brought the Harbeth to mind? Excellent speakers, but what are you thinking that makes you feel you might want to go in that direction? A very different speaker than the VS - also excellent. I think Inna's comments are a bit overstated (a lot?) but not atypical of the audiogon commentariat. What could be said of your TT is that is more than good enough to show improvements in tonearm and cartridges. But if significance is what you are looking for, it is speakers that will significantly change the sound you have now, though an SS amp would probably be better suited to drive you ML, not an easy load for a tube amp.