The Presonus items you mentioned are pre-amps and converters all in one box. If you upgrade to separate pre-amps and converters, you will not need the FireXXX units. The possible exception to this is if you choose converters that have ADAT or S/PDIF outputs and then you may need to use something like the Presonus boxes as a purely hardware interface for those digital formats. In this usage, they would have no influence on the audio quality as they would only be passing digital data into and out of the computer.

I've gone through this thought process as well - and I ended up running up against the thing Boswell mentioned. At first glance it seems as if an interface is just preamps and converters. But perhaps the most important feature is its connectivity options. When I started looking at converters in the price range I could consider, I almost always found I needed the interface to route everything.

I decided to hold off on better converters and spend the money to upgrade the preamps. The pres you mention have been around for a long time and will probably still be in use a decade from now. Can't say the same thing about converters. My impression is that there is more sonic bang for the buck in the pres than the converters as well, but I can't say that I've done careful tests to back that impression up.