This is somewhat of a reversal of opinion for me. I love my 501s — and my first experience of the Hornet was Ray's prototype. My experience was that the Hornet via the Overture DAC (Lan-mod) created a huge, exciting sound through the 501s that stunned me. The Hornet does have the power to drive the 501s with ease.
Listening to the 501s with my iPod 4G 40GB > Hornet (as a properly portable system) I feel that 501s are not the best pairing with the Hornet in this context, and that the HF-1s have wonderful synergy with the Hornet/iPod. The 501s are better paired with the Portaphile PV2^2 for iPod. The PV2^2 brings out the rich mids, warm (if modest) bass and big soundstage of the 501s with the iPod, while the HF-1s sound a bit more thick and sluggish in this pairing and really sparkle with the Hornet/iPod.
The 501s — compared to the HF-1s — with the Hornet/iPod sound rather thin, brittle and fatiguing, compared to the HF-1s which are deep, tight, fast, richly textured, very musical and thoroughly enjoyable with the Hornet/iPod.
The 501s do far better with the Hornet if used with a good DAC (Overture/MicroDAC work very well) than with the Hornet/iPod combo (though I still prefer Hornet+HF-1 and PV2^2+501).
Perhaps we'll see a good, really "micro"-sized DAC someday soon for true portability — and hopefully an iPod with optical out. Why does Apple always have their head in the sand while touting their products as the preferred A/V computers & pocket players? Even my Titanium Powerbook lacks optical or digital coax audio out while Apple hypes it as musicians' and filmmakers' laptop of choice! (End of rant.)