I like that they fit my API lunchbox but I much prefer the API 512 to any of them.

Just because the OSA fits into an API (tm) 500 series rack does not mean that there is the intention to clone any current API product, so the comparison is a bit apples and oranges. It's like saying a Great River and a Buzz Audio sound the same because they both are a 1 rackspace 19" design. The OSA MP1-C sounds similar to a vintage API (tm) unit circa 1970, and is specifically made to sound like how it sounded while the preamp was still physically in the console. I would describe the OSA MP1-C as having a very warm sound with the capability of being somewhat crunchy when the input gain is pushed [another difference from the API (tm)]. It has a gently sloped down top, and a focused/tight/punchy mid and low frequency range.

The MP1-L3 however is quite different, and the goal of this preamp is to deviate greatly from the API based sound (while still retaining the 500 series architecture compatibility)...in that it sounds 'wider' in it's soundscape rather than focused, it's low frequency range is fatter, and it's highs are more extended. The closest preamp that I could compare the MP1-L3 to would be a Neve 1290 (the preamp found in a 1073 preamp/EQ), but of course the OSA is a faster sounding unit being that it is class A/B, and is slightly more 'hi-fi' in it's character.

Again the transformer is able to be pushed aggressively with the output attenuator, a feature on all of the OSA's, and each input transformer (which is what makes each of the 4 models unique) will have it's own distortion characteristic. Of the two OSA's that you are asking about here, it's of my opinion that the MP1-L3 is more versatile, but the MP1-C can do things to the audio that the L3 cannot do. Both are meat and potatoes, and most likely for that reason are quite popular.

OSA has a slightly different design now, but at first they were making API cones with old API opamps and trannys. After hearing a few of the new OSA modules, I surely didn't think they were any better than my API 512's.