Hi
Some people report some success using just the Preamp Tube block but as the SM7b requires quite a bit of gain, I had no success and it introduced a lot of hiss.
A preamp between the two is what's needed, I have never tested a soundcard pre out to the Axe FX, but logically it should work.
Personally I'd sell the soundcard if it's not needed and use the funds for a preamp. I had great success with a DBX286 which is cheap and provides some features the Axe does not, such as a deesser, then into the Axe for effects.
Personally I use a GAP Pre73 Premier (has some nice grit) into the DBX286 for processing only, then into the Axe. I did use the Axe for effects but Cubase provides more flexibility for modulation and reverb. I get a really good vocal recording this way but if I was to sing and play guitar again, I'd happily rack that config up and use the Axe for delay/reverb/EQ etc.
Whilst the Axe is great, after a period of testing I found it didn't sound great as a vocal processor, and the preamp with a good signal and DAW processing was a lot better for recording.
Just to note, whilst the GAP Pre is a nice sounding preamp, the build quality is dreadful so I would recommend buying something else.
There is nothing "premier" about GAP stuff but it does sound really nice.
Mike