Whelp are you going to get to play it? It is not even close to the same thing as a Marshall. They are very dry, clear, and dark in comparison. Due to the power section IMO, the UL at first doesn't seem to have a lot of gain on tap. It does have 4 gain stages on the Lead channel though, so there is plenty there. It's just so tight that it makes it seem less saturated from a feel perspective.
If you are sloppy, you will know it almost immediately, but the amp will eventually make you a better player IMO. I haven't played a JVM, only heard them live, but I would guess the JVM seems more saturated if the gain is maxed out?? Most people seem to try and compare amps to a modded or rodded version of a Marshall, Fender, Vox, etc. To me, the closest thing I could think of is that the UL is closer to a rodded version of a Hiwatt.
What I love about mine, is that it is one of the few amps I've played that can do modern metal tunes, while having great string to string clarity, yet remain open sounding, uncompressed, AND I don't have to use a noise gate even at volume. Even thought the rhythm and lead channels share an eq section, I use the graphic eq to change the tone of the lead channel, which gives me 3 completely different sounding channels. The clean channel is actually really good as well unlike alot of high gain heads. You can do country music with a UL if you needed to.
It really is the most versatile amp I've ever played, even though I'm usually playing metal with it. Also, no matter what tubes I've tried in it, I've always come back to Sovtek KT88's in the power section, and all Chinese 12ax7's sans the at7 in V6, and either a Sovtek LPS or Tung-sol in V1. I used to always run the LPS in V1, but I've been digging the Tung-sol in that spot recently.