Actually, it's not the same:. According to its cnet review: "The 40mm driver is unique to the HD 700, and is not used in any other Sennheiser headphones". (This may have since been rendered incorrect as the new HD660 seems to be a 650 with a 700 series driver in it.) But it's definitely not the same driver as the 650, which uses a 30mm driver IIRC.
From 'What Hi-Fi' : "They use trickle down technology from the HD 800s, with a smaller driver. The drive units also incorporate a new ventilated magnet system, which aims to reduce air turbulence and harmonic distortion that could affect the sound quality."
That said, I did find the overall signature (I've actually heard the 700 and owned the 650) to have more in common with the HD6** series than the 800. For some reason, they didn't seem to me to offer enough 'extra' to justify the extra cost over the HD6** variants. (The 'street' price has dropped though.) They also didn't 'grab' me the way the Amirons did. Although the 'What Hi-Fi' review gushed about their comfort, I found the earcups a little small, unlike the spacious HD6** models and the Amirons. As always in audio, and especially with 'phones in some ways, YMMV. Despite being a Beyer user (for now anyways), I still think Senn makes a great product. The longevity of the HD6** line in the fickle audiophile market is without equal AFAIK and is a testament to the quality of the design.
Over the years I've had 414's (original 2kΩ and 50Ω reissue), 424's, 580's, 555's, 485's, PX100's, and maybe one or two that slip my mind. Good product, but as always, listen before you buy and/or have a good return policy because Sennheiser isn't reluctant to design to various signatures. You may like one and hate another.