Will VPI's Classic 3 sound much better with a SUT?

If you're getting sufficient level now, I don't see any benefit to adding a SUT. If anything, the added gain may result in distortion.

You don't say why you are thinking about adding a SUT to this system.  That would help.  Also, as a rule you don't match a SUT to the TT.  The TT doesn't give a hoot what SUT you choose to use, and vice-versa, but I understand in this case that the Bob's Devices SUT can sub for the VPI cable junction box. (Couldn't any SUT do that?)  Anyway, your Cadenza Black makes 0.3mV (at 1kHz at standard velocity) and your Fosgate is said to provide 60db of MC phono gain. That's a 1000X increase in voltage gain. So that means the Fosgate feeds 0.3V to the Unison Research amplifier.  Most integrated amplifiers do not provide much if anything in the way of linestage gain, so I guess the total signal voltage to drive the amplfier section is 0.3V. I cannot find the input sensitivity of the UR amplifier section, but 0.3V is in general a bit low to drive a typical amplifier to full output. So, can it be that your sound is a bit lean?  Is that why you are thinking about a SUT?  In any case, if the sound is a bit lean and you are turning up the volume control to achieve anything near satisfying levels, then yes, you might be better off with a SUT driving the MM stage of the Fos.

Your Fosgate has 60db of gain. Enough  for most of the popular MC carts.

Former owner of  one for 12 years- great phonostage. Get a different  cart for a different  "sound" Also a longtime VPI Classic user

If you're  seeking  dramatic improvement dump the VPI/Fosgate and open the wallet wider. 

You will hear less tube rush at high volume with SUT-MM setting if you're the type that listens for it.

Needless expense, IMO.

lewm: Your math is off! The Fosgate mc stage amplifies the signal about 10X. So .3 mv becomes 3 mv. That is enough for a typical phono stage. You didn't figure in the gain of the Fosgate preamp's line stage. Most line stages can output several volts. Plenty to drive power amps, which typically need 2 volts for full output.

Jason, I don’t know where to begin. But 60db of gain provided by the MC stage in the Fosgate equates to a 1000X voltage gain, and a 10X gain that you claim would be insufficient for any phono stage ever. 0.3mV multiplied by 1000 = 0.3V

Also, I didn’t figure in the gain from the linestage because I cannot find any information on that and because most integrated amplifiers don’t add linestage gain. So I’m guessing the linestage (which is probably just a volume control ahead of the amplifier input stage) adds no gain. To support that assumption I found that the UR has only one tube that drives its mosfet output stage. That leaves no gain device available for any sort of linestage gain.