Couple of thoughts on doublefour noise and volume...
Noise: Older Double Fours (up to the middle of last year?) are noisy, or rather their power supplies make them noisy, simply because the earth isn't carried through to the amp. I found this very quickly when I plugged the line out to the input of another (earthed) amp and the PJB went eerily quiet! Phil Jones confirmed this was the case, and said that a new power supply would be released (which it has) which would solve this. He also said he would send me one, sadly he never did despite a gentle reminder Anyway, I solved it slightly tackily by running an earth cable alongside the power lead to the negative side of the DC connector.
Volume: The Double Four is remarkably loud (for its size) but I've found it lacking in sensitivity on passive pickups, so I built a 10dB booster amp which brings it up ok. But to give an idea: last year my son borrowed it for a school production of High School Musical to use with his (active) bass. Six piece band in a usual school hall size of a fair few hundred people. The night we went, he asked afterwards if we could hear the bass ok. We had - it was clear as a bell. He smiled and said "That's lucky, they forgot to plug the DI in for the first half, so it was all coming from your amp!"
I've haven't yet compared the Double Four to the larger PJB amps, but for the sort of folk/acoustic stuff I play I've been absolutely delighted with it (once it was boosted and quietened). I've got an 18V laptop powerpack I run it from for portable work which works pretty well, although the battery does cut out at high volumes.
Watching people's faces when you hit the low E on a UBass clone plugged into a Double Four is very satisfying