[i][color=#0000cd][b][size=8]D'oh![/size][/b][/color][/i]
What a doofus! The figures I quoted at the start of the thread were measured against chassis ground, but what [i]really[/i] matters is the potential difference between the plate and the cathode. I actually measured the PD between the cathode and ground when I first took my readings. Subtracting these, I find the plate-to-cathode PDs are all in the range 197V-212V, well within the recommended and design maxima (300V and 330V as far as I can tell).
I also got a very prompt email reply from Fender, who kindly sent me the schematics for the pre-amp board and the back panel PSU. The latter is a completely conventional step-down transformer, followed by a full-wave rectifier and a simple LP filter for smoothing. The output of this is marked as 315V DC, but that is with a primary voltage of 230V AC. UK mains voltage is required by law to be 230V (+10%/-6%), i.e. 216V-253V. It is typically 240V, and I measured ours (with the same uncalibrated meter) as 242V. Therefore the nominal 315V DC supply should actually be 315V*242V/230V = 331.4V. I measured it and got a reading of 336V, only 1% difference!
Genz-Benz will have been well aware of these tolerances when designing for the various markets.
So - I am no longer worried that the amp is frying my tubes. However, I do wonder why the change in performance of the old Peavey tubes. It could well be that (a) they are cheapos, and ( they had an easier life before I put them in the Streamliner.
EDIT: It appears that the letter b followed by a closing parenthesis is interpreted as an emoticon!
Next step will be to fit a NOS Sylvania in V1, then try JJs in V2 and V3. I will be monitoring their condition closely, and this brings me to my final observation, one I am reluctant to make after dropping £349 to own it - [i]the Orange valve tester might not be all that![/i]