Hi.
First of all, welcome to the TalkBass
Zac.
Intermittent noises from any amp is usually traced to dirty FX loop contacts. You can try to eliminate that by using jumper cables on the loops.
The SWR pre tube socket isn't the best of the best either, so retensioning/cleaning may help.
The biggest issue with SWR Fet output sections is that they require biasing every once and a while. As a matter of a fact, almost every commercial Fet amp needs it. It's a simple eough procedure for a competent tech, the instructions can be found on-line. The most noticeable symptoms when the amp is biased incorrectly is that the output distortion increases, and the heat buildup is even greater than with SWR's in general.
When You say that the problem is worse when the output sections are bridged, there's a pretty good chance that the balance is incorrect between the said amp modules. The biasing I mentioned earlier is one possible cause, but there are others as well. A tech will be able to diagnose those better
.
If the wall voltage fluctuates or varies too much, an "ancient" PS design can't keep up and the symptoms You describe are a strong possibility.
Regards
Sam
