I was having a problem with a loud, ugly/uncontrollable sustain on my new rig (SWR Grand Prix/QSC PLX 1602/various cabs. I was puzzled about what was causing this, at various times thinking that the cab or even my bass was to blame. Eventually I realised the problem disappeared if I took the power/pre-amp off the cab. The problem existed with whatever cab I used and by a process of elimination I realised the power amp was responsible. Symptoms are a loud, aggressive, grating sustain particularly noticeable on the low E, Eb and D on the B string, but also noticeable on some other notes. It seems to be some kind of resonance problem. I'm not technically minded and am reluctant to open the amp up in case it invalidates my guarantee. I've spoken to the retailer (Soundslive in the UK). They have spoken to Shure (UK distributers) who have offered the preliminary opinion that the magnetic field from the bass cab is causing the problem. This seems an odd response to me because it suggests they think the amp is unsuitable for use in a bass rig. Given the number of bassists who use them, that seems surprising. They have offered to look at the amp under the terms of the guarantee, but I understand I'd have to meet shipping costs and given their attitude so far I don't have a lot of faith that they aren't going to turn round and say there is nothing wrong with it. Anyone have any comments or experienced similar problems?