A few things Antony.. first, 48khz is not at all a standard in audio for anything aside from video production. Distributable music is all at 44, all commercially available sample packs are almost always at 44. If you're working in music you'll get the cleanest most efficient conversions by going in even multiples. Some SRC's handle it better than others and are near flawless, but many aren't so even multiples is a good way of playing it safe..
One other thing here.. shame on you for coming in throwing around Nyquist theory! I happened to catch that in a thread from a yr ago where you were telling people higher SR's absolutely hold no improvement and are nothing but a placebo effect to those that think otherwise. You couldn't be more wrong and dropping Nyquist theory has no place in discussions about digital audio production. Its only relevance is in recording.
Why you're talking about ability to reproduce frequencies higher than we can hear I do not understand. It's got nothing to do with that. As I previously mentioned it's about raising the upper band limit so that the filters and the aliasing created by digital processing further out of the audible range away from the music. Yes, further than the extra 2.5khz that you get from working at 48khz. Digital aliasing is NOT something that occurs only in the top 2.5k of the frequency spectrum.
I'm done debating this though as I don't have the time and it'd only go in circles from here. To have the stance that nothing can be gained by going above 48khz is to imply that every company who's oversampling multiple times above that is doing it for no good reason and they're simply fooling evreyone with what's nothing more than a marketing gimmick. You know what the developer of IMO the best vst compressor ever made recommended to me recently? Despite his plugin offering up to 16x oversampling, he recommended I just run it without oversampling since I'm already working at 88.2khz where the biggest difference is already achieved.
Ionis - Your ears may be deceiving you.. it's easy to allow ourselves to be tricked now and then. Expectation bias and whatnot. I would strongly doubt your firebox would have anything positive to add from the AD conversion. The cleanest you can get is the source.. which is exactly what is streamed directly into your computer with the usb. Your converters will only color the sound if anything but if you like the sound then go for it I say. The Access team have said so several times and said directly that running it through the analog outs will not give you better sound quality. I admit I was skeptical of that myself til about a month ago but I was able to confirm it and find some distortion to the waveform that I wasn't happy with. I'd rather track the source directly and handle the SRC myself.. then run it out through any of my hardware compression or distortion if I choose while still retaining the original. I have to print to disk first anyways if I'm processing stereo stuff since my outboard units are mono so I wouldn't really want to deal with a source that's gone DA/AD/DA/AD.
Anyways.. are you using other outboard synths or gear? I would avoid the FF400 myself. Duets sound good. A Motu ultralight mk3 would get you good sound.. people are a little clueless as to just how good the converters are in those. People are so easily fooled by expectation bias and believing something's good or not because they read it somewhere. We've done testing on a lot of the top interfaces and the transparency of the Motus in test results (using proper test software) compared to the rest were shocking. As for the Virus, the inputs on it tested much more transparent than my MR816x. Do you have any issue with using the Virus as an interface? Unless you need extra inputs or the ability to work above 48khz I'd use that if it works with your system. I'd bet on the DA/AD quality being better than the firebox.