It sounds like the only weak link in the chain is the A/D converter in the D-8. That's the only thing I'd recommend looking into upgrading. But, if you're going to add an A/D between the preamp and deck, then you should probably at least consider an "all in one" box (like the aforementioned digimod UA-5) that has phantom, pre-amp, and A/D to minimize the amount of equipment you need to carry. The other advantage of the UA-5 is that is has both a mic in and a line in, so you can do simple matrixes as well, if the opportunity presents itself. Good luck.