Ahhh, those are Ampex 351's -
I have one as a standalone pre, and there is some good info on them on here.
The pre's are usually pretty amazing, very 3D, beautiful high end. Not transparent, but not overly colored, either. It's a very excellent sound and very detailed! Mine's modded (from a schematic I found here done by Greg at Electrical) to have an attenuator at the output so I can crank the input and get glorious crunch. It's pretty unreal.
A lot of them need some significant work to get them back to sounding great, hopefully your friend has already done that. I had about $600 worth of work done to mine, which was more than i wanted to spend, but it made is super quiet now and I love it so it was worth it. I think there's only 3 tubes that are actually in the preamp path. I had the heaters changed to DC and took the power switch out of the circuit, because it was bundled with audio wires and contributed noise to it. SO now I just have to plug it in and unplug it to turn on and off.
I have those schematics if you want them, and the guy who modded mine had Jensen write up specs for a step-down transformer to make it not so hot, it was going to cost about $100-150, and is a drop in replacement.
Regarding mic input, the inputs are pretty hot- I can barely turn it above 2-3 before it's in the red with a dynamic or condenser mic. I've seen some pads put in these, and I kinda wish I had one now.
PM me if you want more info- but a search here for Ampex 351 will get you a lot more too! Also, there's info here:
http://recordist.com/ampex/
You're gonna have fun!!!
