External Mic Preamp

The ISA One is a step above just about any interface preamp.  It has a transformer which gives it different saturation characteristics than transformerless designs.  Commonly adds thickness, tho the type of transformer used isn't as vintage as other units (like the Warm audio or API or vintage neve).  It has a "bigger" sound to my ears than my TC Konnekt's preamps (which sounded better than my presonus).
 
Let me say the above difference isn't night and day except in East Placebo-land.  But if you use it for a while you'll see (or hear) what I mean.  It is a high gain preamp (80 dBs with everything to the max) which means you can use your mic farther away from the subject being recorded, which has benefits (as long as the room ain't awful).  It gives you more options and a slightly more refined sound.  There is a lot of little things a pro preamp can do that the garden variety interface preamp can't do or handle.  But it ain't magic, so don't expect it to be.  It is also something you won't outgrow and won't die after a couple of years use.  You might not put it in your will, but you can use it until then.
 
It also has a lot of other tricks.  Most preamps (like the warm above also around $400 but more "vintage" sounding) will also do DI.  But the One has two ins/outs so you can record vocals and your guitar at one time.  Granted, there is only one transformer so the DI goes w/o it when used in mic/DI role.  If used as a DI only it uses the transformer, and makes an excellent bass DI.  There is also an impedance switch, which gives the mic different sounds and really helps w/ a ribbon mic.  There is also a nice DA you can put into it.
 
I reviewed at the "Scotsman Review" web site you can look up.  But the above mostly just sez what it does.
 
It is well worth the money if you are looking for the last couple of inches in sound generation and are interested in learning good engineering.  You don't need it to make quality sound, but it helps w/ the job.