Just bought the little dot mk3 I am very impressed the hd650 sounds fantastic. The bass is very punchy soundstage is huge and I still have to burn the tubes in properly. The Stx amp and dac were a little boring without the external amp. Now I know why the hd650 and tube amps are well regarded in head hifi. :-)
Glad you are enjoying your setup.
With the 650's its all about the extension , once you dig the extension it turns into something else :)