BMF's recent post is just chock-a-block loaded with good information. One thing that might get overlooked in this flood of audio goodness is information on how to setup a basic frequency-response testing kit.
If you've ever questioned whether one of your mods actually sounded better than another (or that one of your mods sounded better last night than it does this morning), a test kit can help confirm or reject your listening impressions. And these are the docs you need to read read:
3. Room Equalization Wizard Setup Summary:
4. Phantom Power Supply & Panasonic WM-61A Mic Slide Show:
For about $30 Canadian, including shipping, you can get the electronics components you need for the mics and phantom power box. The software, Room Equalization Wizard, is free and written in Java, so it works on a number of OS platforms. You'll need a soldering iron. If you've never soldered before, this isn't a bad first project.
This setup won't match the one that pros like Tyll uses, but it has saved me hours of tail-chasing and wheel-reinventing when it comes to T50 mods.