One of my favorite microphone preamps is the Presonus MP20. Not only is this a pretty mic preamp, it is relatively low-priced and can be modded to make it sound oh so hot!
A few years ago when I was running my own recording studio I came across the Presonus MP20 microphone preamp. I think I happened across it on ebay when I was looking to add a preamp to my collection. I picked it up for a great used price and started using it.
I later heard that it was possible to modify this preamp. I did a lot of research and even spoke with a fellow (Rick Naqvi) over at Presonus about what op amps and transformers sounded best in these units. I found out that Presonus origninally put Jensen transformers in their preamp models but later switched to a lower cost transformer. I opened my preamp and I found two Jensen labeled tranformers. Bingo! I bought one off ebay that already had the original Jensens - this was good news so I wouldn't have to do that mod.
There are a few different types of opamps that can be swapped into this unit. I decided to go with Burr-Brown opa627ap opamps (it takes 4 of them - two per channel in the MP20) because they seemed like the popular choice. I ordered them relatively cheap from the manufacturer and swapped them in. The preamp sounded great! It gives a nice phatty tone and is good for either microphone or direct as the source.
Here's what the opamp looks like:
Get this - The opamp mod can be performed to other Presonus gear that are similar to the MP20.
Try modifying the PreSonus M80 8-Channel Microphone/Line Preamp to get 8 channels of fat sound! 
There's also the PreSonus Eureka Pro Recording Channel which has one channel. 
Click the pic below for more specs:
Presonus also made a unit called the VXP that is supposed to be very similar to a MP20/Eureka: 
There are some other opamps that can be used for the preamp mods. People say that the Burr Brown opamps give a bigger, beefier, bottom end sound - nice and phatty. Analog Devices AD-797AN opamps can also be used. These are said to be similar to the stock opamps except a bit better in most aspects like faster transient response, greater clariety, clearer attack, better detail and more. Another option (and quite affordable) is the Linear Technologies 1357 (Digi-Key full part number: LT-1357CN8-ND). These are said to be very quiet, with super noise and distortion performance for very sterile accurate sound.
So, when I was looking to do this mod, I scoured the internet for all the info I could find. I'll list the links of some awesome sites that have tips and audio samples of the modded preamps. If you want to know how something sounds with the modded preamp, let us know and we'll see if we can record a clip for you to check out.
Click the underlined links:
John Rice's site: This site has a lot of great info about the transformer and opamp mod. There are sound clips and even a step by step walkthrough of how to install the transformers.
Home Recording Forum: Some talk about the mods
