Ray,
The D6C deck was never intended to record music, though, it does a nice job with it.
They were intended to use for gathering news sound bytes in the field, and for convenience. The Sony mics match the power supply well enough, but are not that good of a mic for quieter, sensitive recording; and pretty much suck for anything loud and amplified. They are intended for a reprter to shove in someones face for comments.
There is an inherent issue with the mic preamp, and its "power-in", and that is that the power-in makes it too sensitive to use with outboard condensor mics that require larger voltage. When that signal voltage hits the preamp, it overloads, or does what we call brickwalling. This brickwalling is an electrical voltage overload of the preamp circuitry, and it presents itself as a truncated wav form, and in distortion in the drums and bass in loud amplified music or passages.
So, to counter that, we opt for the line-in rather than the mic-in. The line-in bypasses the overly sensitive mic preamp and is a far cleaner input source.
The problem with this is that the line-in has no built in amplification, so the incoming signal will be very weak, and your s-n-r will be very high in the noise side -- not what you want with analog.
So, to counter the weak signal, you use an outboard preamp to give gain to the signal prior to the line-in input of the recorder.
With cassette analog, its crucial to get this gain amplification in the mastering process due to generational degradation in analog.
In real loud music, you might not need any amplification for going line-in, and might be able to run the D6 line-in at levels that require a bit of attenuation below level 10, but quieter music and ambient nature recordings will need to have a bump up in the levels. Back before we had portable mic preamps, we'd run line-in at level 10, and attenuate down from there, depending on how loud the music/impulse was.
At any rate,... almost any quality outboard mic preamp will be more suited for music an quiet circumstance than the onboard D6 mic pre; Its not bad, but its not great.
Church Audio has a very small mic preamp that he sells. Give that direction some serious consideration. It might cost a bit more upfront, but, you'll save a step in the upgrade bug.
Yu can get in touch with Chris Church, here at ts.com. Hes a member in good standing.
This is one of his ads in our retail section:
http://taperssection.com/index.php?topic=130844.0