I think you are right about the 701 being between the 650 and the 880.
I consider the 880 slightly brighter than 'neutral', the 650 slightly darker than 'neutral', and the 701 right on. But we aren't talking massive differences here. I do think the 701 has more bass control - they seem to be able to deliver the same weight and impact as the 650, however don't seem to have that continuous bass weight that the 650 seems to lend to every instrument - the 701 is just as good at getting out of the way as it is at getting in the way in the bass department, if that makes any sense. I'm a real fan of the 701 for classical music - I'd never heard anything that rivaled my 650 in terms of tonal accuracy, and had also never considered it anything other than neutral. The 701 has very quickly become my reference standard for all things a headphone should do, even including headstage that the 650 is famous for. Of course it isn't perfect, but it for the first time showed me the things that the 650 doesn't do, and even with upgrade cables is never going to do. But it's hardly night and day. They seem to be aiming at a similar tonal point.
I do feel however that there are aspects to the sound of the 701, relative to both the 880 and 650 that can't quite be described in terms of frequency response, or bright or dark etc. It has the ability to resolve transients that gives it a lightening quick speed - we often tend to consider a headphone with peaky treble to be 'fast', however the AKG is fast in the true sense, without an artificial frequency response - I'd be willing to bet that the driver could do a better square wave, when appropriately driven, than either of the other two cans mentioned. Who cares? Well, no-one really, but it is an interesting measure of transient accuracy (cue stuff about flat voice coils or whatever). It also has the ability to project instruments in a beautifully articulated way, again without relying on artificial detail (cue drarthurwells to tell us about the 'accuracy' of the SA-5000, with of course achieves this via jacked up treble response which 'aint accurate however you swing it (though it I suppose might be 'fun'). Hearing an entire brass section playing in front of you is a wonder with the 701 - very hard to describe, but seriously 'hi-fi'.
Regardless of whether you find it to be your cup of tea or not, I think any serious head-fi'er owes it to their hi-fi'er inside to audition them in a quiet environment - we all know that noisy meets don't leave an accurate impression of open headphones.