In my opinion, Focusrite is the best of the affordable bunch of interfaces, with the best preamps and converters. My audio bud reviews equipment for a living, and he agrees. No doubt, RME and other brands have nicer stuff, but not in this price range.
I second the notion that the preamp quality is subtle. Most super expensive preamps don't impart fidelity so much as pleasing imperfection through tube distortion. Maybe I'll upgrade preamps someday, but that's a long time off.
And converters are also subtle. They could be important if your instrumentation and mixing is exceedingly organic, or if you make lots of roundtrips from digital to analog and back to digital for using hardware effects during mixing. There are conversion losses each time, even if your converters cost $4,000/channel.
In my opinion, unless you mix all of your effects from analog hardware effects racks, or unless you record high fidelity jazz or classical performances with natural reverb only, your interface is holding you back. The more synthetic your music, the less it matters over a baseline standard that it sounds like you already have. I'd consider improving your acoustics, work on better positioning your speakers, refining your low end, buy new virtual instruments, or possibly upgrading your monitoring.
