Looks like a well thought out design. Two quick things, 1) you have to like the 12au7 tube sound and 2) the toroidal transformer does not always equal the best sound but they are very good indeed.
We manufacturer a 6SN7 preamp for around the same price as the extreme but we use the best parts that we have heard (our opinion & experience) and we are point-to-point wired.
For those who recommended the Quickie - it is very good for the price but does not compare if you are looking for the best sonics.
Happy Listening.