I use Emg active 81s into my saffire pro 24 intrument input and they sound great, i own 2 DI boxes which ive compared and i couldnt tell any difference atall.
If your using passive pickups then i can see different inputs/DI's giving slightly different results.
quote from Brandon E (another user on this forum)
''I don't use a DI box, I use my Saffire Pro 40 with the instrument and pad switches on. I used to have a Countryman Type 85 but I did a shootout and could not tell one damn difference so I sold it ''
and here is a clip of those DI's and they sound great.
http://dl.dropbox.com/u/3597430/18vs9.zip
If your DI tracks sound bad through the saffire you may need to take into consideration the pickups your using, guitar/wood, string gauge, new strings? old string? blah blah blah
If your DI tracks sound bad through the saffire then its likely they will sound bad through even a great DI box like a Countryman Type 85.
there was a shootout on the forum not so long ago, most users couldnt tell what Di was the saffire and what Di was the coutryman.