Low level amp hum

I have a Tungsten Crema Wheat that recently has developed a low level hum after it has been on for about 20-30 minutes. It's inaudible when I'm playing but will gradually fade back in when not playing and left on. If I turn the standby switch off the hum will stop. Turning the standby switch back on and the hum will gradually come back.
FWIW, I substituted the stock JJ GZ34 rectifier tube for a ANOS Sylvania 5V4G to get a bit more warmth and sag. I've had the Sylvania in the CW for about 3 months. The hum started a couple of weeks ago.
The other tubes are Marconi 6V6Gs and RWN Neuhaus 12AX7s, which were all NOS when I installed them.

What's the best way to diagnose and fix this problem?

Thanks!

 

I'll try the tube swapping thing.
In the meantime, I think I may have found at least part of the problem. I'm almost always playing through the Bright channel, Input 2, and don't usually turn up the Normal channel volume. I tried turning up the volume on the Normal channel and it started to hum and the hum got louder as the volume increased. By the time I got to around 10 or 11 on the dial, the CW was howling like a herd of cats in heat!
I have a feeling I'm gonna be giving Adam a call. But I'll try swapping tubes and see what happens.