I have one of these, its fantastic
There is some really good information in the manual on trouble shooting.
I had a very similar problem caused by a faulty valve in position v3 which is a 12AT7
PROBLEM: Signals are present and working normally in the PREAMP
OUT {W} and XLR (PRE position) {AA} jacks -AND- Automatic Bias
display {T} shows all output tubes are good (solid green LEDs), but
the Power amplifier lacks power and punch, sounds different than
normal, or produces no sound from speaker or no signal is present
from XLR (POST position) jack {AA} in ‘SILENT RECORD’ mode.
SOLUTIONS: Power amp driver tube V3 (12AT7) has failed. Replace
V3 with a new tube.
I had to do this after half a dozen gigs and its been fine ever since.
As an aside, Its 40 odd years since I last had a valve amp, it runs to me very hot but fender says this is normal and the fan doesnt cut in untill its almost at nuclear melt down temperature.