GEC Brown Base impressions:
From 45 minutes' listening, at 6am, this is a gorgeous thing. It's warmer and richer - tubier, even - than the 5998 but the detail is all there. It seems to bring the focus back to the mids, which really suits the HD650s. Also, the bass thumps like the stock Crack with a Mullard 6080 but with the extension and tightness of...well, there's no comparison with anything I've heard.
I've got to get to work now but looking forward to some extended listening this evening.
Edit: For example, Aphex Twin - Flim. There's a whole other level of richness to the synth decay which has never come through before. Boards of Canada...crikey.