Yes, I have tried re-seating the valve...no difference.
Re the Silvaweld, the audio circuits are fine, but I have the remote control version (too lazy to get up to change the volume, sit down again and fiff and faff finding the sweet spot again ;-) and the problem is a "chip" on the remote control board needs replacing but suitable parts aren't available.
The buttons on the front panel also go through the remote control board...so it's toast at the moment.
Out of interest, I did see another SWC1000R for sale - this time with a battery power supply. There seems to be quite a few variations of the pre-amp.