Jamin, you don't have to worry about damaging your M50s, because your ears will give out long before they will. A more significant question would be why you have the idea that you'd need more maximum power capacity than the 875 would provide in standard operating mode. The first point is that you can rely on the rated 140 watt figure as being accurate, since all amplifiers sold in the U.S. have to be rated in compliance with FTC regulations which require that the rated power with at least two channels driven be sustained for at least 5 minutes continuously(unrealistically severe for home use, of course). No manufacturer is foolish enough as to violate the law in regard to power ratings, so this is one of the very few things in audio where we should be confident that the advertising is true.
If you haven't studied it before, the S&V lab test
of the 875 verifies its power capabilities.
Bridging an amplifier sends twice the voltage into the speaker, and because of Ohm's Law power is proportional to the square of the voltage, so theoretically, doubling the voltage would increase maximum power capacity by 4 times. Inefficiencies don't permit this in practice and roughly 3 times is said to be a typical result(note that the S&V test said only that it was over 300 watts).
Keep in mind that most of the time your M50s use about 1 watt at a comfortably loud listening level. Much more is used on brief peaks, of course, and enough headroom should be provided for as much as 20dB over the average level on highly dynamic source material, such as some classical recordings or an occasional pop item which hasn't been dynamically compressed. In the vast majority of setups receivers rated in the area of 100 watts provide plenty of headroom. Although it's merely the statement of a truism, sometimes there's a failure to realize that unused headroom is simply that: unused.