Anatholi,
The TFPro has an output level of upto +22 dBu - this is pretty hot, so it may be overloading the convertors without clipping itself. The Apogee has a jumper setting for max input level - you could check this, it can be set to +24, +20 or +5dBu. Clearly +5 would be distorting a lot of the time - that would be the 'consumer' level setting. If it is set to +20, change it to +24, that way, you can't overdrive it with the Mic Pre.
Once it's out of the Apogee and into the Mixtreme, you shouldn't have any problem, once it's in the digital domain, you shouldn't have to change anything. So I'm guessing the Mixtreme should be set to unity gain.
I don't have experience of the Mixtreme , so I can't tell you what settings to use, maybe others can chip in. For myself, I never attenuate or change the SAW inputs, I just twiddle the mic gain until I get a reasonable signal.
The other thing to remember is that, especially if you are recording at 24 bit, you don't need to get the levels too hot to get a good sound. Just hitting -8 to -6 on the SAW meters is usually good enough. You've got a pretty high class chain, no need to worry about maximising levels too much.
Dominic