A friend of mine just picked up one of these Acoustic AB50s from GC today. I went over to his place to check it out, and noticed that it had a bad hiss, and a crackling sound coming from it. My friend didn't notice the noise until I pointed it out, but once I pointed it out to the guy it was obvious to him.
We switched out instruments and cables and checked it out with a number of different combinations, and it just had this bad sound to it. I checked out a few reviews of this practice amp -- including one here at talkbass -- and noticed that this is a common problem. I just wanted to warn anyone who was thinking about buying one of these amps that they are apparently hit or miss. 
http://reviews.harmony-central.com/reviews/Bass+Amp/product/Acoustic+Amplification/AB50/10/1
