Your question is probably mute as the B3031A's have not been produced anymore for a number of years. I can say however that the B2031A's have been quite reliable for many people. I've had several pairs, but switched to Mackie's as they sounded better. There is also talk however of the B2031's being discontinued soon.
I can't speak for the JBL's. I've never used any of them.
Eric