Haven taken the light engine apart to play with one the light engine can only drop brightness of the video signal by having dirt and dust in it.
The light engine doesn't "make" light. It makes and passes the image by making the image via a bazillion mirrors on the DLP chip, adds color using the color wheel bouncing color off those little mirrors whose light, brightness or otherwise, comes from the lamp/bulb shining through the color wheel.
The means that the image is made, how all those little mirrors are controlled and how the right colors are reflected properly and at the right time is amazing.
From what I can tell if you have a color wheel going bad it is going to show up with your colors, not brightness. Or it won't work at all like those I've seen whose light engine wheel actually breaks part of the wheel and then the light engine doesn't spin.
The mirrors, and there are a couple of them, and lenses in the light engine can and do get foggy, dirty, etc. There's a couple of threads and youtube videos for how to clean light engine parts. BUT if MIT replaced yours I would never suggest cleaning over free replacement.
The last lens, the one inside the TV cabinet itself that shines the image on the mirror in the back, it can get cloudy, can be carefully cleaned, and it is changed with the new light engine.
Obviously it was something in the light engine. Just pointing out the term "light engine" is a bit misleaded. While probably not 100% accurate either it probably would be better named if it was the "image engine".
E.B. White said, "I arise in the morning torn between a desire to improve the world and a desire to enjoy the world. This makes it hard to plan the day."