Jet City amps are just plain awesome. I have the 50 watt combo and I replaced the preamp tube with a JJ in V1, and a 1960 RCA Grey plate and left the rest stock - It toned down the treble and the RCA in the second position seems to give it a little more punch.
I also have the 20 watt head and that thing rips! I put a JJ in V1 RCA Grey plate in V2 and the V3 stock - some times a 12AT7 works great as an inverter before the power amp section because of the transduction - I did this in my Mark 4 and it made a difference in keeping the bass articulate.
Rolling Tubes is a lot of fun!!

http://www.youtube.com/watch?v=0marWEdsk6k - link to my JCA20H Demo.

All the best,
Joe Pinnavaia
www.guitarlessonsbuffalony.com