Pro-Ject Extension 12 to SME Model 10

Pkelly1504, it will be very interesting to know why you plan upgrade have you found negative SQ in Xtension 12?

I have P-J RPM 9.1 and thinking my next upgrade: Xtension 12 or Signature 10.

The best recommendation I received are: Brinkmann (Brado, Osis), Clearaudio (Innovation, Ovation), VPI (Prime 21 and Signature).  But these cost a lot new, and need expensive dust cover.

Based on price point and top level X-line, Xt12 seems very high end, being weakest link in your system might be not valid I will investigate this if I ware you. It is possible need spend a lot money to get better sound from you already have..., not wise!

Also consider upgrading power plug from wall wart to box use detachable power cord, in case you haven't done that.

Also look for upgrading tonearm to better model (Breuer, Graham, Jelco are good example), tonearm can make significant improvement.

Good luck

@knight7m , You should upgrade your list with the Sota Nova Vacuum. It can also be had with a beautiful dust cover!

Agreed mijostyn5.
I like know what SQ differences between Sota and VPI.   I dont like tube sound, its moody and put me into sleep.   I like SS sound but similar to Krell: definations, resolution, extensions, neutral.

I have a Brinkmann Bardo and a SOTA, either are excellent.

Having owned previously VPI TNT6HR and Technics SP10mkII, I now own and would agree on Sota. There a used ones out there because they've been around 30+yrs, they're HIGHLY upgradeable, and if SME was something you like, SME arms are one of the most common combos on the better Sota tables. 
FWIW, I had Nolas as my speaker when I owned the VPI & Technics tables. My system page details some of my impressions of the changes. Cheers,
Spencer