I'm trying to decide between using an ASUS Xonar STX soundcard, an M-Audio Prodigy HD2 soundcard or an M-Audio Profire 2626 FireWire audio interface.
My only concern is the musical sound quality.
I've tried looking at the specs but I'm having trouble understanding what I'm reading as I'm not very technically minded.
Can anyone tell me which one is most likely to give me the highest quality music.
The ProFire 2626 would work for music playback, but aren't 26 channels a bit of overkill?
For listening to music, any small FireWire audio interface like, for example, the Echoaudio AudioFire2 would work just as well.
All FireWire interfaces have in common that they are optimized for recording, not music playback, so you won't find such features as built-in high-end headphone amplifiers.
As for the STX and the HD2, the STX has slightly better numbers, but in practice, these differences won't be noticeable.
(In other words, if musical sound quality really is your only concern, this would point to the STX, but if the HD2 had any features that you need or even might need, it wouldn't be the wrong choice.)
I've had experience with equipment from M-Audio (mainly their monitors), and they are very good performers at a very good price. Of the two M-Audio devices you are looking at I'd go for the Prodigy 2. Simply for the reason that it doesn't use firewire. Firewire is still common for studio interfaces, but desktop PCs are rapidly moving away from firewire and in a couple of years when you replace your PC you might find no quick way to connect any firewire device.
USB interfaces have a much better chance of being around in the long-term, making them a better investment. If you're just after playing back music, rather than recording, I'd look for a good stereo USB interface that works with default drivers. I use one from HRT, but there are many others. For Audio playback all of the modern buses (PIC; PCI-X; Firewire; USB; Lightpeak?) have more than enough bandwidth to cope with stereo. Far more important is the implementation and it's longevity.
If you want to record, that is a different matter and I'd look at one of the Focusrite Saffire interfaces. I've got a Saffire Pro 24 with two very good headphone outputs for recording/monitoring and it is great for my uses - it ignores two bits of advice I've just given (it is firewire and uses proprietary drivers) but I considered these before I bought it.
I deliberately haven't mentioned the Asus because I have no experience with these. Many people seem happy with them, but there are also a few threads from people looking to upgrade. You'll have to read around.