Some folks have complained about the treble spike, or treble peakiness in the HD700, this is true.... but to any degree more than similar complaints of too much treble or too sterile sounding regarding the HD800? I don't think so.
There is a great argument to be had regarding the HD700 in your collection, and that argument is that regardless of how high or low your opinion of the HD700 is (and mine is very high), I have yet to come across anyone that feels it is in any way similar to the HD650 or HD800. Many good reviews on these cans, and I think a good case to be made about some potentially positive attributes:
1) Better bass than HD650, cleaner and more articulate, and while maybe not quite as deep as HD800 in absolute terms, a bit more punchier and certainly more present in terms of the rest of the spectrum than with the HD800. I won't go so far as to say that is plain fact, but I see a very high degree of corroboration on these points.
2) Not as amp fussy as the HD800 or amp demanding as the HD650, so if you enjoy and celebrate the Senn brand, then I would argue the HD700 is the most versatile in terms of smart phones, tablets and laptops, and other direct headphone connections that are not dedicated pieces of headphone equipment, and yet when you do put an HD700 on anything that is known to work well with either the HD650 or HD800, I get the sense that most agree the HD700 comes to life on those chains as well. So you have a unique situation here where the HD700 should be happy to follow either HD800 or HD650 on any given piece of equipment, but at the same time do better on equipment that does not quite work with the other two.
3) Give the HD700 plenty of time to break in and settle down, give yourself plenty of time to not be so obsessed with the dreaded spike, and there is a very good chance you will join many people who feel the sound and comfort of the HD700 is world class and though not as spacious and detailed as the HD800, it is "musical" and "versatile" and "dynamic" enough to deserve a space on the top shelve of any collection.
At $999, even if money is not a driving concern, I would be hesitant to make the jump, but at $650, why not! Also, if you happen to be near a Best Buy Magnolia or any audio shop that deals with Senn, there is a good chance you can give them a try. I use mine with a Dragonfly 1.2 and love it. I have also listened through the HDVD800 and think they sound great. And finally, they are a bit smaller and lower profile than the HD800, so when you are not in the mood for opposing pie plates that press on your upper jaw line, you can instead opt for the most ergonomic headphones on the planet, the fabulous HD700 : )