The low and high inputs are not gain related labels, they are tone related. The low input allows the full frequency signal to be amplified, while the high input cuts off the low frequencies, making it a bright input.
As for the loss of lows overall, that may be sign of off value coupling caps in the amp. Even when working correctly, I know that some owners of these amps find them to be too bright, especially the reverb/trem model.
I'm sure some of the other guys here will offer their thoughts on this question, but technically, I'd see if the shop had checked the electrolytic coupling caps when the amp was being serviced.