And to think I've never auditioned the HD650...or any other Sennheiser set, for that matter.
Some say the HD600's better, though, less "veiled"...but still pretty dark/warm overall.
All I know is that if someone lends me the likes of the HD650, LCD-2, maybe even HE-500 and HE-4...I'm going to prepare myself again with another SR-Lambda + SRM-T1 setup, and the battle of "most glorious midrange" shall commence! (And then someone would show up with a Koss ESP/950 and a Stax Pro bias adapter to try and decisively sweep away everything else...)
As for dullness, that term makes me think "treble with no sparkle", like the MA900 with no EQ to bring the treble in line. If that's the HD650, I wouldn't like it for sure. The MA900 has the advantage of being cheap ($150 for me, $110 for some other lucky git, $200 and under in most cases); the HD650 doesn't, especially after Sennheiser's recent price-fixing push.