Basically, what I want to know is what are the main differences between cheap/expensive audio interfaces in terms of sound quality? I don't really care about the extras on more expensive models, I just want a basic interface with one guitar input so that I can record with as little quality loss as possible.

I'm currently recording using the line-out on my Spider IV direct through a cheap 1/8" cable (using an adapter) into my sound card (SB X-Fi Extreme Audio), and I can't help but think that the cable is sucking some quality out because it sounds better straight into my headphones, especially with distortion.

Currently looking at an M-Audio Fast Track USB interface http://www.dv247.com/computer-hardware/m-audio-pro-tools-recording-studio--85479

tl;dr: Will the M-Audio Fast Track record with similar quality to a more expensive model?
Periphery, love that shit!
The Guitarport is twice the price of the M-Audio and the only difference I can see is that it models amps/stompboxes... and I have a Line 6 amp to do that anyway :/

Any clues on audio quality?
Periphery, love that shit!
Any interface will be a vast improvement over your current setup.

What effects the quality in cheap vs expensive interfaces are the preamps and converters. I'm sure you get the preamp thing, as that's also what's in guitar amps that help make them good and expensive. Converters (analog to digital and back) make a difference as well. People will pay way over a grand for just a few channels of conversion. Cheaper converters won't be as accurate and transparent as more expensive ones.

When you're in the price range you're in, it doesn't make THAT big of a difference sound quality wise. Now there's a huge gap between paying $100-$200 and paying over $1000, but for what you're looking for, fast track quality will be just fine.

I would, however, recommend getting a UX1 or UX2. You'll get better sounds using amp sims than you will with your amp, and the UX line comes with Pod Farm. Or you could just get a fast track and then download (legally) whatever you want.
After looking at the fast track link you posted, you'll be better off with a different interface. That one can be a bit noisy. The fast track MKII will be a better choice, or as I said, a Line 6 (The GX is cheap but good)
Thanks a lot for the help... I don't really have a problem with the Spider's sound quality for now because I'm saving up for an Axe-Fx (probably 2nd hand) in the coming months, which is the main reason I want a decent audio interface. As I'm spending this kind of money, will it be worth bumping up to an expensive one? I won't be making studio albums quite yet, but given the Axe-Fx's price and acclaim it would be a shame to lose too much quality from it.

Also, I've just had a look at the MKII and it seems like a much better choice, so thanks for that
Periphery, love that shit!
I own a Focusrite Saffire 6. Amazing quality and features for the price, I highly recommend it.
Anything Line 6 (Guitarport, Pod, Toneport) or most M-Audio interfaces will suit you fine. I use Guitarport and use Gearbox/Guitar Rig for my tones. Soon I'll be switching over to M-Audio because of the versatility (more inputs, MIDI, etc)
The Line 6 appears to have less noise at higher levels than M-Audio I have heard. But Line 6 pre-amps don't have as high of output. These are from personal experience. The Lexicon interface I had though had the best control of the ports and input selections, but is considered
a little out of date, but can get just as good quality and is cheaper (these days) than both the Line 6 and M-Audios I have owned.

If I bought a interface over again in this price range, I would probably get a Focusrite Saffire 6. With that said all the interfaces in this price range are pretty similar and it won't matter what you buy.

The price range I am referring to is 2 inputs, phantom power. Roughly $150-200.
I like the MKII, but at the same time, it is different than its previous version in that the second input is FIXED at guitar level input. That means you can't switch between line or instrument level, so if you want to use it as a line in, tough luck, it's going to turn out bad. Disappointing, I wanted to try recording my amp with the line out.
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