Understanding Audio Interface Preamps

Many interfaces use a self-sensing combo input.  You can plug a mic or 1/4 inch jack into it.  In theory, running a mic signal that has already been brought to a line level shouldn't be run through a preamp, but most line/mic jacks won't affect the sound if the preamp gain is not engaged.  In practical terms, you ain't going to hear a difference.
 
The main attraction for external preamps is the fact they can use nice transformers that help round the sound off and you get more gain, which allows you to back off the mic from the talent.  Transformers, to me, are the sound of recording, and most of the stuff I grew up listening to was, of necessity, sent through big-ass transformers.  Big and round really works, to my ears, with the sharpness of digital capture.  Unfortunately, good transformers are expensive - about $100.  So even transformer coupling the input or output is not viable for price-point interfaces.  The best buys for preamps are the Focusrite ISA One and the WArm audio WA12.  The ISA has a more modern sound, while the Warm is more vintage - thicker.  Each is about $400, which is a lot to spend and isn't going to make or break your sound.  If you get one and think you'll hear a world of difference, you're likely to be disappointed.  The instrument, performance, room and mic are more important.  It will help, even if those other things aren't great, but can't fix those things.  However, once you start putting all the lead instruments/vocals/overdubs through a nice preamp you'll likely have a lot more separation of all your instruments and a bit more depth to your recording.  It all adds up.
 
The other thing about a high-gain preamp is you can back the mic off.  This works better the better the room, but at 18" little movements of the head don't make for such dramatic changes of timbre as at 6 inches, and when a passage goes from soft to loud you have some built-in cushion as twice the distance = 4 X the drop in energy.  It is natural compression.  And if you do overdrive the preamp, the transformer will help soak it up, saturating the sound instead of doing the IC crackle and break up.  I've never broken up my Neve portico channel strip (although I haven't intentionally tried) which uses giant rails and tons of iron and generally high components.  It never has even sounded stressed or strained.  I have pushed it and it only sounds smooth.  I can't say that for cheaper interface preamps and other hardware I've used at home. 
 
Spending the money can help your sound, but only you can decide whether it is worth it or if you are ready for it.
 
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