WD88VA - hiss, hum and interference

[wiggers]

I have a WD88VA factory-built for me in 2008 that I use for the treble in a bi-amped setup. A few months after I got it a capacitor blew, taking out a valve, which John Caswell kindly fixed for me. It's been fine since then.

Just recently, however, the hiss has increased a bit and I've started picking up interference from a PowerLine ethernet device nearby. The mains for the WD88VA comes through a UPS with an AC-DC-AC converter, so it isn't getting in that way. It only seems to be on the left channel and it is independent of the volume setting. I've swapped the valves around and cleaned the contacts but it doesn't make any difference.

I've also started getting a noticeable hum from the second (solid state) amp in my bi-amped setup but only after I've turned on the WD88VA. The SS amp has recently been serviced so I'm a bit suspicious they mucked something up, but the retailer swears it was all fine when he gave it back to me.

Any thoughts on how I can work out what's happening?