A piece of advice - ask this question on the tape op and gearslutz messageboards - http://www.tapeop.com/ and http://www.gearslutz.com/board/ - both have a 'diy gear' section and have world respected designers happy to answer questions to newbies. As regulars on here may agree, mudcat isn't the best for knowledgable advice on high end audio...

For that price I (for one) would buy one if it truly is as transparent as you say. Don't bother with the switchable impedance - it makes a huge difference to DI'd guitars and high impedance lines (guitar amps for example) but hardly any noticable difference to low impedance mic signals. Make the gain high enough for ribbon mics - eg the Coles 4038 as a benchmark for the amount of gain needed (lots!). I know its a tall order for someone doing this on thier own, but add a digital output card (a third party one?) and I'm sold on what you've described.

Why do you say transformers are bad for linearity though? I was reading this last night - http://www.prosoundweb.com/chat_psw/transcripts/rupert.shtml - admittedly Neve used his own designs of transformers but still...