Shoot !!!
if you think the interference from your cell phone is bad on the PA80,try putting it on top of, or near to, your guitar tube amp head. You wont miss ANY calls - I assure you of that !! LOL

I don't know what exactly happens - but I've seen this a number of times - the amp freaks out - like the cell signal is being routed thru the higher frequency portions of the pre-amp section. The result is that the amp just goes ballistic. Wierd....

Steve