To Doug Blackburn,
Between a single Belles 350A Reference in stereo mode and a pair of Belles 150A References in mono, which is "better" and/or how do they differ? The cost is about the same on the used market.
I would probably opt for the bridged 150A Reference amps -- there's a small benefit to having separate chassis and two power cords compared to a stereo chassis and single power cord. In addition, the bridged amps will have twice the slew rate (voltage versus time) of a stereo amp because of bridged mode. If the stereo amp will swing 40V in "x" microseconds, in bridged mode it will swing 80V in the same time interval. That makes response to transients lightning-fast, which is audible.
One concern for the bridged configuration is the speaker load. If your speakers are rated at 6 or 8 ohms and don't have more than one low-impedance dip or more than one or two combinations of impedance and phase that cause the speaker to be difficult to drive, you'll be fine. If you think your speakers are difficult loads, you may want to ask Dave Belles directly (through the www.powermodules.com website) about your speakers' impedance and phase characteristics to make sure they will work well with bridged 150A Reference amplifiers. Dave Belles is the owner and chief designer of Power Modules, the maker of the Belles amps.
Finally, there is another issue with bridged stereo amps that may get overlooked but needs to be commented on: both amps have to be identical. Ideally, they should be made close to the same time (serial numbers should be fairly close together) so that the two amps contain the same components and circuits. Often manufacturers change topology or components over time, sometimes due to obsolescence, sometimes because something new comes along that performs better. I don't know if there were any factory updates for the 150A Reference, but Dave Belles should also be able to tell you that, since he keeps track of everything that goes out of the factory door. . . . Doug Blackburn