Vic, Regarding DIY preamps, It's easier for me to classify how I focus, and what I do as:
a. Work with new circuits or kits that copy the original circuits using new, used or NOS parts. (Good)
b. Get the original product (crazy expensive).
c. Get new products that copy the old, and add some new tricks (very expensive, but rock solid)
c. Get a broken unit and fix it. (still expensive)
d. Get broken studio products that no one pays attention to. (the very best)
So for me, it all started with the RCA handbook and the Neumann Bottle Mic used at the 1936 Berlin Olympics. The point is, the answer to great recordings have been around for 70 years, and the circuits are simple and similar.
In my opinion, the cheapest, most used, most famous, and least well known is the preamp in the Scully 280. Banged-up these go for @$100. This is a combo tape mic pre that includes 4 or 5 transformers in the audio path. You'll need to strip the unit down and keep the playback amp card, the blue connector, and the transformers. If you get a good one intact your lucky, but you will need to rig a -22/+22volt power supply to get it to run. You can make a power supply out of 12v APC batteries which take the noise down to almost nothing. "This is" the preamp that was used on countless hit records and was inline after bill putnams mixer, and I think the teltronix la2a compressors at muscle shoals, but nooooo one pays attention to this. The sound of this preamp is rich and powerful, and can drive headphones, DAW, or a vintage amp into classic distortion. It has lots of color because that's what happens when you connect a guitar or mic(coils) to a multi-transformer based mic pre that has transformers so large, they can't even fit into what's out there today; Not to mention they're too expensive to make today.
I'll be posting more dirt-cheap DIY stuff in 30 to 60 days. Sorry for the delay, but I'm getting busy at my day job. Lastly, I don't buy anything that I can't turn right around and sell for close to what I payed for it.