The Fun's discrete amp section is more than sufficient for the HD650. It's clean and neutral with power in reserve and drives these cans with authority. It holds its own against the b22 according to Milosz's A/B comparison thread and its been pointed out by another user (sphinxvc) who has owned it in the past that he found it equal to some big ticket gear and only bested by the RSA Apache and a 3 channel b22 under meet conditions. It has the smooth neutrality and black background Audio-GD owners identify as their house sound. For the HD650, I wouldn't be surprised if it performed as well as your C-2.1.
I suspect the Fun's amp gives the sense of these headphones close to their best with solid state, short of a balanced topology. For these headphones, the Fun is not shamed at all by the ridiculous power reserves of the Lyr. Where it falls short is in the general difference between the "wire with gain" signature of the Audio-GD ss and the musical presentation of the Lyr tube design. The Lyr has more impact, and the imaging presented is phenomenal. I once read that a good indicator of a properly driven HD650 is the sense that sound is originating from beyond the driver's position. This can be heard through the Fun, but on the Lyr it's more apparent and even prevalent at lower levels. The tubes I have in place are also quite neutral and impart a greater sense of space between instruments, so when all these things combine it produces a perfect storm of slam, resolve, imaging and an analogue quality that makes the Fun's solid state appear flat, cold and clinical in comparison.
At this level, it may boil down to preference, and I can certainly appreciate how the ss signature with it's seemingly faster and snappier response would appeal to those preferring a clinical sound for their critical listening. For me, the Lyr's musicality and the synergy achieved with the right tubes have heightened what I enjoy about these headphones so much, namely its ability to engage me in the music without obsessing about glaring faults.
