I just purchased a pair of Etymotic Research ER-4P canalphones, and I was testing it with my available audio interfaces. I have a first Generation Creative Labs Audigy, an M-Audio Transit, and an M-Audio Firewire 410.
The Audigy was noisy, and not all too nice. I tried the Transit, and the sound was much cleaner, no perceptible noise, etc., but the bass seemed really diminished.
I then tried the Firewire 410 (headphone outputs), and it's superb sounding. All the bass is there, very clear sound, no noise. It's such a flexible soundcard too, with eight analogue outputs that can be assigned to two headphone monitors. Edit: I can hear a very slight bit of noise when I turn the headphone volume controls on the actual device up to the maximum. I'm not too happy about this!
I'd love to try this with an amp now... but has anyone else noticed that bass really seems to suck on the Transit? I'm wondering if maybe I'm just used to a distorted and uneven frequency response on other soundcards or something.
I also have a firewire Audiophile in stock, but I'm supposed to sell that, as much as I'd like to rip it open and listen to it.