One thing that this amplifier invariably needs is power tube bias adjustment. The Champion 600 Reissues that we have seen have had the 6V6 power tube biased for too much power dissipation. The correct power dissipation for a 6V6 is considered to be between 12W and 14W (12W for vintage 6V6s and up to 14W for some of the newer 6V6s). The Champion 600s that we have seen were biased for between 15.5W and 16.5W. This is simply too high and does not sound optimal. We have found that the installation of a 200 Ohm to 240 Ohm 2W resistor in series with the factory installed 470 Ohm resister in R10 will achieve approximately 12W to 13W power dissipation.
There is much more that can be done should you wish to take it further. This amplifier, like all single ended amplifiers, would greatly benefit from hum reduction achieved through a DC filament supply. You will find that there are countless threads on countless message boards dealing with modifications to the Fender® Champion 600 reissue and if you wish to carry things further we suggest that you search out some of these threads.