saving bringing up the wretched water under the bridge, when it comes to M and avalon and the likes, understand that these people are in business to make money and if you want one of their products you should go buy one. There is enough AMAZING stuff out there designed by very smart guys that is no longer in production which nobody has a conflict with which you could build which will sound every bit as good or better than the things you mention. There are a few companies *in business* exclusively ripping off neve and api and others, so it seems pretty fair game to go after that stuff. Other companies have original designs or are under then impression that they have original designs and we should respect their intellectual property rights. I dont know what youd need to search for, but the politics of this have been discussed ad naseum which is probably why nobody wants to get into it anymore. The group has agreed more or less not to fuck with some people.
If you are dead set on building a tube preamp, there has been some talk about a few guys doing an rca pre and I think there is some info on the k&K audio site for a kit with boards.
dave