Unless you are interested in some of the features unique to the 7 series, I don't think there is enough of a difference when doing PA taping to warrant the price anymore. Stage-lip/onstage stuff might be another story and even then there is the usbpre2 (again, missing features, but sounds the same and is cheaper).
I really don't see any difference in recording PA, stage lip, or ambient/nature recording for that matter, when it comes to the M10 versus the 722(line-in).
I think with non-PA recording there is a greater importance or opportunity on retention of soundstage cues and micro-detail like plucks or clicks that occur in instruments but can get wiped out or smeared when processed through a lot of gear and then blasted into a reverberant room. The natural decay of a cymbol or drum head sounds different for example. Maybe it's just my experience, but I love the sound of a mandolin, trombone, or drums when I'm taping close by versus much further back in the room. Truely; ymmv.
I once listened to a comp between the Nagra 6 and the 788 and I found the best way to find differences between the two was not to listen to the chorus singing, but to listen to a priest or someone else coughing during a conversation to the left of the congregation. With the Nagra, I could start to make out words, with the 788 I had more trouble.
Heck, that 7-series ADC chip is going to be old enough to drive before people realize it ain't worth thousands of dollars...
if it's preferred, why change it (they didn't in their usbpre2 which came out a year ago).
But on a serious note, I don't think many are buying a 7 series box for the sound quality necessarily anymore as for it's dual media, fail over power, et al. It's the features, cause otherwise if you wanted that sound, you'd pick up the usbpre2 which has the same adc and preamps as the 788 at half the price of a
used 722. I've thought about selling mine, but letting go of that feature set isn't something I want to do at the moment.
I don't think the M10's adc stage is a slouch, it's a nice unit, in many ways I think I'd be happy with one (well, a D50, but basically the same thing), especially with the price, but the features of the 7 series have saved my butt more times then I care to count.