Here's the good news...The pre-amps sound fantastic with a microphone plugged direct to the mic in port. This is the first portable I have used that allows me to use a dynamic mic without going through the external pre. Levels are strong, I am not detecting any noise using the Rode NT3, NT4 (stereo), or a cheap dynamic Apex 870B.
Now for the rub. The internal (topside) mics are no-where close to what I'm getting with the externals. Hiss using the built in mics is similar to the hiss I get with the Edirol R-09 internals. That would make them good for loud sources such as bands, but not so good for low level recording. I do hear background hiss with the built in mics. It must be the mics (not the pre-amps); I hear slightly more hiss in the left channel (right mic) when I'm in XY configuration. When I switch to wide configuration (right mic is now right channel) the hiss is stronger in the right channel. Again, nothing major, but I would not expect the D50 to be a strong contender with its built-ins.
Considering the exceptional job it is doing with every external mic I try it with, I'm feeling very positive that the D50 is going to meet my personal needs.