"Increasing output by 3 dB (the smallest volume increase the human ear can normally discern) requires a doubling of power. When bridged for mono, MC275 conservatively makes 150 W regardless of load, but as that translates to only 3 more dB, you likely won't get a big difference in output."
Things are never that simple. If you read the original post, he's clearly looking for better sound quality, not more volume. If you have a big speaker that's very efficient, the drivers still need to be controlled, and that takes power. Wilson is a good example of this. In this case, though, the speakers are not that efficient. To be honest, I'm surprised he's getting good results with just the 1 amp. Adding a 2nd amp will yield much better sound quality at the same volume he currently listens at.
In most cases, bridging a stereo amp will give more power at the expense of sound quality. In this system, I don't think this will be the case for 2 reasons. First, the 275 has a history of working well bridged. A lot of people run them as mono's and are very happy with the results. Second, he needs more power. I'm willing to bet that if he called McIntosh, they would recommend not using just 1 275 to power both speakers. They may even say 2 bridged is not enough. Either way, another 275 will be a significant upgrade, and not a tweak.