I also use a 640P and agree with you Airborn that it sounds slightly on the warm side of neutral. I'd also add that the 640P has a surprisingly deep soundstage.
I've found the 640P sounds more neutral when used with slightly brighter sounding turntables and cartridges, although lately I've been listening to a Grado Gold on a Technics SL-1210 and it sounds very satisfying.
Compared to the Hagerman Bugle, The 640P is slightly darker, resolves a bit less detail, has a much deeper soundstage, sounds more musical and conveys more emotion.
I just looked in the manual and see no figures for impedance and capacitance, I find that curious too.
Dave