You should look at the Ray Samuels Emmeline II Raptor. I did not like the Sennheiser 650 at all until I got that amp. You can tailor the sound a bit by tube rolling and Ray is very easy to deal with. The Single Power has bit more tube lushness than the Raptor. You can read reviews of these products at
http://headfi.com/
http://headfi.com/