I too find the HAP-100 amp section to sound very similar to the DAC-100. With my Sextetts, I find the DAC-100 amp section has a little more bloom in the lower end (not necessarily cleaner, just more energy), while the HAP-100 seems to give a bit more edge to the treble transients. The differences are very minute however, and it's possible I may just dismiss that due to headroom or placebo after more listening.
For those who like numbers, here are a few measurements:
DAC-100 headphone out, max volume, 1M load, 60Hz sine wave at -1dB = 3.4V
HAP-100 headphone out, max volume, 1M load, fed the 60Hz from DAC-100 = 7.8V
so that's a bit more than double the voltage swing, which equates to roughly 6dB more headroom (more like 7, but we'll go with 6 for simplicity and calculation sake).
The output impedances are wonky and seem to wobble a bit. At best guess, there's a resistor running in parallel with the headphone load, and I've measured them both to be roughly 16-17 ohm. Considering that they recommend headphones with 100ohm impedances or higher, I might theorize that someone on the design end literally took the 8:1 rule of thumb to heart.
There must be some other resistor or interaction in there though, since the HAP-100 is affected more by different loads. With a 560ohm resistor load the DAC-100 output measures the same, but the HAP-100 measures 7.5V, and at 33ohm the numbers drop to 2.3V and 3.6V (note that this is into distortion range of the HAP-100 since we hit the current limit of the amp). So we can see that the effect of lowering load impedance is more drastic on the HAP-100, which is usually indicative of a higher output impedance, but there are other factors at work here too.
edit: hmm some preliminary number crunching does not work if I assume a "simple" resistor in parallel and in series with the load... I actually wind up calculating a negative output impedance or something like that
Edited by Armaegis - 11/10/12 at 8:54am