VPI Prime or Prime Signature?

Hi folks, I’ve got a VPI Classic and Lyra Delos cart. I would like to upgrade to a Prime or Prime Sig. If I go the Sig. route I would stick with the Delos. Or should I get the Prime with a costlier cart? Privilege problem I know, but would either be a significant upgrade over the Classic which sounds great? Phono stage is LKV, pre is AR ref 3, amp is AR Ref.75, speakers are soon to be Martin Logan. Thanks for your input!
You can never have too much turntable. Or cartridge. But while the cartridge is a wear item a good turntable is pretty much forever. A good arm though is even more forever than the table. 

You are getting into a price range where it makes sense to really think and plan your analog front end long term. You can keep going for the convenience of the package deal. Turntable makers will love you, this is hugely profitable for them. People doing this never figure out what's what. How could they? Still this might make sense especially if you're pretty sure your next table will be your last table.

Or maybe you set your sight a little higher. In that case then it will make a lot more sense to start looking into the arm and table as separate components. The $3-5k it looks like your looking to put into the next package, I can just about guarantee you can find an arm that is so good the arm alone will be at least as big an improvement as the whole package. For sure the Origin Live Conqueror you could get for that money is way beyond anything VPI has to offer on any of their tables.

Its a little more work this way. Whether or not its worth it depends on your ability, desire, and long term plans. Food for thought.

Aa a fellow VPI user, you're at the hair splitting level where your hard eared cash is invested in questionable ways.
I don't hear the value of the Signature over the regular Prime. In the context of an entire system, Phonostage will be the game changer. After that, I would go Prime/Delos first.
Subjective in the end, you don't hear what a table/cart is capable of until you run it thru the best phonostage available.
Considering you have a tube front end, complete it with an appropriate tubed phonostage to get more tube magic. At that point, any upgrade becomes meaningful.
If you already tried tube, and it's not your thing....never mind.
I would go with the Prime Sig with the Fatboy gimbaled arm.
Not to be a stick in the mud but have you thought on another option and buy a nice used table and move up even further? Or similar to what I did with the build up of a vintage Garrrard 301-401 or thorns equivenant. Rim drives can be very addicting if done right and your In  the budget range to do just  that, with more to upgrade too down the road. 
VPI dealer here****

Which Classic do you have?

Your setup is quite nice as it is.  In terms of "significant upgrade",  you are really not going to see that with either option.  If you want minor changes (note separation, tighter bass response, slightly better upper and mid range detail), the Prime Sig will give you that.

If you like your current cart, and are just looking to have something different with better overall sound, then the Prime Sig is what I would suggest (plus the Prime Sig Rosewood looks flat out awesome).  If you want a bigger change in the overall presentation of sound you are getting, then the Prime with a totally different cartridge or new phono stage update would likely give you that.

Either way, you will still have a really, really nice setup.