Just got the HPA-21 on loan, and I pitted it against a 300B tube amplifier (it's a local brand so it's no point saying the name). I just posted my impression in the LCD-X thread here: http://www.head-fi.org/t/684394/audeze-lcd-x/3270#post_10296711
Of course, Bakoon sounds magnificent with LCD-X but IMO, with types of music I listen to, it's still a step behind this 300B amp. I also tried it with my HD650 and the result is the same.
Some info about equipments used in this comparison (nothing fancy, actually):
Source: MBP with lots of HD music (24bit and DSD) and player is Audirvana (with and without iTunes integration)
Dac: Ayre QB9 DSD
Cable: MBP to Dac = Nordost Blue Heaven
Cable: Dac to amp = HGA DNA
LCD-X Cable: Cardas Clear
HD650 Cable: SAA Equinox
Music of choice
- Country (Alison Krauss, Willie Nelson, etc)
- Quartet/Trio Jazz (Holly Cole Trio, DK: All for You, Dave Brubeck, etc)
- Standard Jazz (other DK albums, Michael Buble, and many others)
- Many titles from Stockfish, Chesky, etc, that sound good.
I mostly listen to vocal albums and don't listen to any classical or orchestra kind of work.