I'm skeptical that any recording interface with its own preamps will be better than what you have with the A&H. You might want to look at a preamp if it's going to be a good, solid addition to your sound pallate. I'm running a PC with an old SRC console by Allen & Heath, and it's a bit noisy by today's standards, but it delivers just barely enough good, clean gain to fire up a ribbon microphone for vocals. This is good. Some cheap preamps will fall short of this. Something like a Great River MP-1NV will bring new sounds to the table, at something like 10 times the cost per channel. It's a very coloured preamp that can sound agressive but still focused to make the mixing challenge a bit easier - very good for rock guitars & such. Big sound. I'm lusting for the 2 channel version. Others like the Grace are more neutral, or transparent sounding. Neither choice is right or wrong, they're just different, and there's a ton of other choices. Allen & Heath is very respectable stuff. You have to remember that a lot of professional live recordings have been routed through a large format Allen & Heath console before going to the recording truck. There's better stuff available, but they're not new at this.
For starting out, you might get more mileage from using the A&H, and concentrating on good A/D conversion, good mics and good recording practices. The room will also come into play.
For converters, you might want to look at something like an M-Audio soundcard, or Lynx, RME or others that can deliver a bit better performance for more $$$. The M-Audio stuff is supposed to be very solid and reliable (I run a Delta 44 and I'm very happy with it for my needs, for now, and they're cheap) but others are supposed to have a more robust midrange. Every factor adds up, but many interfaces with built in preamps will not compare with your A&H, and the converters are probably no better than M-Audio.
Beyond that, what mics you get into for what you're recording, the sound of your recording space (the room) and your mic placement techniques will be far more important to the overall picture. In general, condensers are good for acoustic instruments, cymbals and percussion, and for including room sound. Dynamic mics are good for loud instruments and electric guitar amps and stuff, and they tend to reject the room a lot more. If your room sounds bad, dynamic mics are your friends. Ribbon mics can be made to sound great on just about anything, but they're very fragile and generally gain-hungry. Also, very high quality dynamic mics are a lot less expensive than very high quality condensers and ribbons, but there are a lot of respectable budget condensers out there. Each different model and type will bring something new to the table, so you may want to look around. A pair of instrument condensers will work well for acousitic guitar and piano. Shure SM 57 is a good starting point for electric guitar, but if you can swing the cash for a better mic,
the SM 7b is a deadly vocal mic that can do double duty on a lot of different sources, and it's awesome on a guitar cab. There's also the Sennheiser 421 or 441, Beyer M201 and others. A good vocal mic can be practically anything, as long as it does what you want. The right choice for one vocal might sound bad on someone else, so it's important to experiment, and helpful to spend a bit of time reading about general mic palcement techniques to get you started.
Good luck,
sl
