They may need time to burn-in, but so far they sound quite shallow. None of the excitement and power of the AH-D1000.
Efficiency is 103 dB (probably /1 mW), so they're somewhat quiet straight out of the soundcard at maximum loudness. Nominal impedance is 28 ohm, which can be somewhat risky for Mac owners (the Powerbook here damaged a set of cheap Sony earbuds @ 16 ohms, killed a couple cheap Technics supra-aural headphone sets with 24-ohm impedance, and damaged a set of Panasonic circumaural headphones with a 22-ohm impedance), though they'll probably be fine.
The CMoy amp couldn't drive them to decent loudness or power before the noise floor became noticeable. They are much less CMoy-friendly than the AH-D1000.
Size-wise they're a bit larger than the AH-D1000, even though the diaphragms are smaller - 35mm against AH-D1000's 40mm. Isolation is way better than with the AH-D1000 (probably 6 dB difference or more), but as the AH-D501 played quieter with the PCDP, the AH-D1000 isolated from street noise better. Even when not playing, AH-D501 are in the "what did you say?" class, whereas with the AH-D1000 on the outside world and voices are discernable.
AH-D501 have the "AKG-style" self-adjusting headband (and plastic rails), which is a bit of a problem as the fit is somewhat loose. They really need more grip - the seal isn't that good on a medium-sized head. Fine over a cap though.
So, in summary, for someone looking for a cheap companion to the Denon AH-D1000 or AH-D2000, the AH-D1000 (or AH-D1001?) might be a better alternative (or just lugging around the AH-D1000 - they're not an electrostatic set either). The price difference isn't as large as the difference in liveliness.