Hmm, I may have spoken too soon - after the first couple of hours playing at home, the bias indicator lights are showing a fault and it can't be adjusted to the correct condition. Reading through the manual, it's almost certainly a power valve issue, though I don't have any 6L6GT valves around to verify this. Fortunately, Kenny's have been helpful to deal with, and are arranging an exchange. I did suggest that I'd be happy with just replacement valves, but apparently Polar don't stock them, they just exchange whole units.
Still, it does sound good, and failures in the first few hours can happen with any valves, so I'm not too worried as long as it gets sorted...