You can burn-in a preamp all by itself. Does not have to be connected to anything, although doing so might help a little. Sometimes delivering a little bit of output power (even if into a 47k resistor) works the output stage better than a no-load.
More info on voltage- and current-mode burn-in found in the FryBaby manual.
jh
More info on voltage- and current-mode burn-in found in the FryBaby manual.
jh