I use the Graham Slee Solo. It's very good. Many people on here don't consider it to be as good as some other alternatives at the same price level, but I have become very fond of it.
It may have a tendency to highlight the top end. I'm not absolutely sure about this but using it with the Beyer DT880 may not help the 880's (re criticisms). Having said that, I have had some extremely low grunts appear on the 880's via the solo so it must have a lot to do with my sources too.
However, it is very good with the Sennheiser 650 and I saw some demo guys from Sennheiser using the Solo to drive them at a Hi Fi meet in the UK.
I hear that the 701's may be on the 'lighter' side down below so the Solo may contribute to brightness.
It's quite a lot of money so I think that it is essential to hear one with the headphones first since the combination in UK pounds is somewhere around £700. That's a lot of dosh.
To me, the solo is a very "punchy" amp, with pretty detailed highs. It was a great match for the HD650, and help to alleviate a lot of it's purported "slowness". I'm not totally sure what the K701 sounds like though, so I can't really help you there.