The best bang-for-the-buck upgrade I ever got was buying a
Weber ceramic Blue Dog speaker for my Blues Jr. I'd strongly recommend calling Ted Weber and talking over what tone you're going for. He'll tell you which speaker will do best for you.
The second best upgrade was the Weber Mini-Mass Attenuator. Being able to get fully saturated power tube distortion without going deaf or making the neighbors call the police is fantastic.
About the reverb... well, it's not so hot. There's nothing wrong with the reverb tank itself - I've talked to Accutronics who makes good reverb tanks and it turns out the Blues Jr. already has one in it :smack: The "Blues Jr. Mafia" thread on the FDP talks about this periodically. Bill Machrone- the resident guru there on Blues Jr. mods has a website, including this section on the reverb:
http://mywebpages.comcast.net/machrone/bjr/bjreverb.htm
Enjoy!
Chip