FAS has a "secret sauce" for their inputs so unless an audio interface has the same set up it won't sound the same. I'm not saying an interface will not sound better or worse but it won't sound the same. The algorithms matter but as I said, and as FAS says, they are only part of the equation.
http://wiki.fractalaudio.com/axefx2/index.php?title=Axe-Fx_II_connections_and_levels
I recorded DI signals from the Axe Fx II via SPDIF, and then through the DI of a Steinberg UR824 Audio Interface, its smaller sibling UR28M, and then through the DI of a Universal Audio 2-610 tube preamp. Then I reamped all these DI signals via SPDIF through the Axe Fx II. Tried it with a variety of models, from Fender clean to Plexi roar to Engl high gain. Results:
- The Axe Fx II DI sounds reamped exactly like playing through the amp models right away. Doh! Who would have guessed...
- The Steinberg UR28M DI recordings sound much worse when reamped. There is a lack of detail, depth, a high end roll-off, etc. The interface's 500kOhm DI is obviously not doing a great Job.
- The Steinberg UR824 has a 1 MOhm DI and performs much better: Recordings made through this DI sound reamped almost like those made with the Axe Fx II's DI. They are slightly - really only slightly - brighter sounding, because the upper bass / low mid range is a little thinner. However, that was easily fixed with just slapping the good old Waves Renaissance EQ on the track and boosting that range by 1-2dB. Apart from that, regarding detail, resolution, high frequency range, depth etc. the reamped tracks were just as good as those through the DI of the Axe Fx II.
- Reamping a recording made through the 2.2 MOhm tube DI of the Universal Audio preamp was even better than reamping the Axe Fx II signal. As if a tube pre worth approx. $1000 per channel adds a nice sheen to the sound. Again - doh, who would have guessed...
One thing that was very important for the comparison was to look at the peak and RMS level of all the DI signals and adjust the levels of the non-Axe DI signals to the level of the Axe Fx DI. The Axe Fx II DI signals were typically around -22 to -16 dBFS, if you run a hotter signal through the Axe for reamping, it will quickly distort in an unpleasant way.
