As many of you already know, the Valve jr is a simple two valve circuit.
It's also a modders delight, which has spawned a whole sub industry of kits and forum threads dedicated to getting the most out of it. Now I'm a guy who likes to pull everything apart and get the soldering iron in there(more on that in a future post). But I haven't done anything more than swap out the valves.
I happen to own two incarnations of this amp, one each of the combo and mini stack.
The only reason I changed any tubes out was because I knocked over my combo and one of the tubes became rattly.
I've owned a number of valve amps over the years, and these are the only ones that I ever noticed a significant change in the tone after a good warm up period. After about 40 minutes the top end opens up providing a crisp tone and a little more volume.
My first one came along because I didn't have an amp at the time and heard the buzz on the interweb about them. I picked it up from a local 2nd hand dealer for about $180 and haven't looked back. The stack I bought new from an instrument dealer in the CBD, can't remember how much I paid for it, but it was less than retail.
They are cheap, but have earned their reputation as good little recording amps.
I use an FX unit with them to get my reverb fix, but other than that I play them as is.
One of these days I'll join the big boys again, but for now I'm running in 5watt stereo.