I recently got a couple nice mic pres (API 512c and Chandler TG-2) and I'd say it's pretty subtle. Mic positioning is (of course) still just as important. And your choice of microphone has a far larger impact on the sound than the preamp in my experience.
Now I'm fairly new at this and have a lot to learn, and definitely need to spend some more time experimenting, but I'd look at upgrading/adding microphones rather than outboard preamps given you have a decent interface. That will make an instant and very clear difference.
From what I've seen a good sounding end product is more about people with experience and good ears making the right decisions at every step of the process, as opposed to a gear competition. From the choice of instrument/amp/cab and it's positioning in the room, to the choice of microphone(s), to the mic positioning, to the EQ/compression applied, to the hundreds of choices made in the mix etc. etc.
That said, I guess it's one of those things you won't really know until you try it yourself (you could always rent some gear for a few days and try it out). And perhaps you need the proper recording space, overall audio chain and monitoring facilities to really be able to discern the nuances.