I've never heard of anyone using an Acoustic AB50 with a double bass; it appears from the literature that it was designed for acoustic bass guitars. I use my Acoustic AB50 as a practice amp with my upright guitarrón, which has K&K bass guitar pickups, and with a K&K Pure XLR pre-amp. Sounds great for that use; no problems at all. (I initially thought it had a buzz, but that turned out to be because I had it positioned too close to some furniture.)
As far as the lack of a gain control on the AB50 is concerned, my pre-amp does have a gain control, but the K&K pickups seem so hot that I have to turn the pre-amp gain all the way down anyway. This combo is so loud with my set up that I have no doubt I could use it in a small venue, if I ever got that good.
Others have commented on an air-rushing sound from the cab, but if mine has it, I can't hear it. (Neither can my wife and her ears are 10x better than mine.) From a comparison of photographs, it looks like they may have vented the cab since the earlier models were made--I dunno for sure. The photos in the online manual seem to show a wide, horizontal slot at the front bottom, but it looks like it's filled in, like a plug in an unused switch position on the dashboard of a car. Other photos on their website and current photos at retailers' websites make it look like this slot is wide open, which mine was when I bought it.
I had been concerned that, as a combo expressly made for amplified acoustic bass guitars, it might not reproduce the highs well enough to enjoy using it to play mp3 play-alongs, but I tried it at GC before I bought, and the ride cymbal, chord player, and soloists come through very clearly and cleanly--with my bass added to them.