The Pete Millett Hybrid is currently my favorite amp. It's great. However, it might not be for everyone. (Note, the amp I built is somewhat upgraded from the basic specs -- linear power supply, all the electrolytic caps are Elna Silmics and most have doubled capacitance from the spec, output caps have triple the capacitance, and there's an Alps Blue potentiometer.)
There are three reasons why I say it might not be for everyone. First, the sound is on the dark side of neutral. Either you'll love this or hate it. For comparison, I think a Meta42 is slightly on the bright side of neutral, even with Burr-Brown OPA637 and EL2002s. The Millett Hybrid is noticeably darker. However, this makes it a really nice amp for Grados. In general, it drives Grados very well. The second reason is that this is definitely not a low-distortion amp, i.e. there is a definite tube signature. With low-impedance cans the "tubeyness" is fairly restrained, but with higher-impedance cans the "tubeyness" is definitely present. If you look at Pete's article about the amp in AudioXPress, you'll see that at 2V output you're looking at 5% total harmonic distortion, mostly second harmonic. Either you'll love this or you'll hate it. I love it, but if you're a solid state man or dislike a lush sound, this is not the amp for you. The third reason that it might not be for everyone is because with high-impedance cans it can't go super loud. You won't be able to drive HD600 to ear-splitting volumes with this amp, but ask yourself if you would be listening that loud anyway. Even the Etymotic ER-4S can't be driven super loud, but that's probably a good thing, since it's hard to judge volume with those cans.
Anyway, despite those caveats, it's a stunningly musical amp. Not neutral, but totally enjoyable. Awesome low-level detail, even at low volumes. If you've ever heard a Fisher 400, it's probably closer to that sound than to most of the other headphone amps I've heard. I like it.
