Doesn't matter what you use with these mics but you must get a preamp with a really good phantom power to run the QTC50 to their full potential. For this reason, I've paired up my Earthworks M50 (same as QTC50 but with tighter specs) with their already discontinued pre amps. I think it's called 1024 with zero distortion technology. Was very lucky to get one new one back before they were discontinued.
The Earthworks range of mics are really different to normal large condenser microphones in the way they pick up detail and transients. It's very accurate to the source and captures close to how your ears would hear something. You can hear the detail down to the very subtle nuances of a source. It doesn't color the sound in anyway but materials tend to sound clearer more hi-fi but not colored in anyway. Sometimes pureness/exactness isn't always what you want in recording a performance so adding some colouration via pres/tape saturation works well for the Earthworks range of mics.
Depends on your workflow, but you can also go in very clean and than colour the signal during mixing.
Just be careful though cos detail can sometimes be not very flattering to a source - in these situations I normally go for a larger diaphragm microphone like the Neumann M149 to add some tone to the source.
I normally run some 2nd 3rd and tape saturation with my Cranesong HEDD 192 to add an extra dimension to the Earthworks mics.
Earthwork mics are weird in a good way. Nothing like it imho.
I would also try the apogee duet with the QTC50 to see if you like the sound.
Mixing Engineer www.acoosticzoo.com