Eminence Swamp Thang.
Also, here's a good rundown of various speakers for the Blues Jr: http://billmaudio.com/wp/?page_id=295
The Eminence Swamp Thang makes any Fender amp come alive with volume, tight bass, and clear highs. I'm sure it will allow a Blues Jr. to be cranked without breaking up in an unpleasant way.
And BillM mods are worthwhile, if you can handle a soldering iron. He's done a lot to help sell Fenders, since he's corrected a lot of issues that inexpensive amps suffer from.
The BillM tone knob made my Fender Excelsior into a much better amp. The stock tone switch was a design flaw.