> a good design or kit for a DIY headphone amp
I have nothing to say except what's been said: Headwize.com
> something really nice sounding for my personal enjoyment.
That is utterly subjective (also highly biased by headphone impedance, tone, background noise, listening level, and source material). Listen to anybody's opinion, but don't believe any of them.
My unbelievable opinions:
Some differing opinions are due to the varied impedances and power demands. Some sweet tricks work well at 300Ω, less-well at 32Ω, or OK at 1mW and not at 200mW. Since impedance, sensitivity, and acoustic levels each vary about 10:1, the difference between one situation and another may be 1,000:1.
If you can afford power and heatsinks, use a good op-amp followed by a fast power BJT emitter follower, all run from at least +/-12V rails (Senn 600 often need more voltage, more than any good opamp). Very wasteful, but serious power, and clean and simple. I wired one on perf-board.
If you want fun and funky, get a TV V-sweep tube and use a power transformer for an output transformer. Power output is ample in most impedances, THD is low and simple, and you could literally wire it on a breadboard (wood board for cutting bread) with some brass tacks for solder points.
Ordinary hi-current opamps do astonishingly well for mid-and hi-Z loads, and are often satisfactory at 32Ω, though they are not sexy.
Remember that a headphone amp IS a "line amp": headphones come from that tradition. For 100-300Ω phones, use your Neve class-A or Jensen 990 module connected as 150Ω. Put about 20-30 ohms in series with the output to reduce headphone melt-down.
You can wire CMOS inverters in mass-parallel and get adequate power with a very round sound.
There is also GainClone. If these things are so wonderful driving speakers, they should be killer at driving headphones. And they are certainly simple.
-Zeus