Maybe a EQ pedal, just boost the lows and low mids a bit?
This works... up to a point. The problem is that the Blues Junior has small values for coupling and tone stack caps. So increasing the bass puts a bigger signal into the amp, which the amp knocks down again. It's like turning up the water pressure into a small-diameter opening. You increase the flow until you hit the limit.
At low volume, you can use an EQ pedal to tailor the tone however you like. When you turn up, however, you run out of headroom faster and induce more harmonic distortion (not the pretty kind).