I've been modding my JCA50H since I got it because I wanted a Marshall but didn't want to pay Marshall prices. The first thing I did soon after buying it was to swap the screen grid power resistors for ones that would suit EL34s and switch to those tubes. It was an improvement, but I think you need to get really loud to really notice major differences from power tubes alone.
Then I did enough preamp mods to align it with pretty closely with the SLO100. But it still didn't have enough of the congested upper mid Marshall crunch that we all love regardless of whether or not we know it. So some cathode-bypass adjustments here and some coupling cap adjustments there got close, but once I swapped out the cathode bias resistor on the cold clipping stage, it was there. That was R45 on the PCB, and 10k is the right value.
Mine isn't a whole lot like that anymore, though. Changed out the faceplate, power section, most of the preamp, etc. But I have a total of under $400 in it even after the mods and buying it new, so there's that.
It's essentially a license-built version of the Soldano Hot Rod 50+. In terms of budget Soldanos, the SP-77 preamp gets closer to the Hot Rod vibe, but the Jet Citys are in the same zip code at least. The biggest differences I've noticed are that the taper of the Jet City's gain pot is significantly faster, and they tend to be a bit brighter and less clear. The shortcomings can be largely mitigated by swapping out the stock tubes, and overall they sound great for a few hundred bucks. I don't play my JCA22H a lot, but it's the amp I plug into when I want 'that sound'.