I had the SR-303 (which is, for all intents and purposes, sonically identical to the SR-404, even according to 6Moons Audio, which is saying something), so I can answer that question.
The Lambda Pro is much rawer, detail-intensive, and forward than the SR-404, in my opinion. The SR-404 has more sub-bass, but is less punchy and aggressive, and it sounds more diffuse and euphonic/colored than the Lambda Pro.
It's sort of like the difference between an American muscle car (Lambda Pro) and a Japanese tuner (SR-404).
Edited by Tus-Chan - 9/26/13 at 2:20pm