Now that it has been a couple of weeks since the initial rebuild I've had time to play the 135W twin a bit more and do a bunch more research.
This model twin does not have a bias adjustment on it - the bias is fixed, and the only adjustment is a "tube matching" potentiometer, which doesn't add any value when you buy a matched quad of tubes.
The tubes are running "cold" - a measured bias current of 20mA. This amp needs about 40mA bias current, so there is a lot of cross-over distortion. It is good sounding distortion, but the amp is not giving the traditional clean Fender tone - it is always a little "dirty".
When I get some time over the next few weeks I will rebuild the tube matching circuit into a proper "blackface" bias circuit. Just needs 3 resistors changed and rewiring the hum balance potentiometer.
I use a Eurotubes "Pro One" bias meter:
https://www.eurotubes.com/store/pc/bias%20probes.htm
I'm still not thinking I'll keep it (it weighs north of 80 pounds) and 135W is ridiculous, even if they are "Fender Watts".