When you wire the amp for mono, the gain control only controls that channel. You could in theory adjust each channel separately to change volume, but that would probably get old very fast :^) so a preamp would be a better approach. A so-called "passive preamp," basically a separate stereo volume control with source selector switch, is easy to make and can be inexpensive to buy - Schitt make one for $49.
Note that the bridging puts the outputs in series, so you lose the ability to drive 4 ohm speakers - the bridges outputs can be set for 4 watts into 8, 16, 32, or 64 ohms.
