From what I've heard, the through-the-preamp or direct-to-amp question is like many in audio: both can be good, depending on the usual factors. Impressions I've gotten indicate audiophiles are split about 50-50 on the question, both, no doubt, with good reason. Meaning, aside from any meaningful technical questions such as that as raised by JohnnyR above, the only real route is to try both. My preamp maker and dealer feel, for my equipment, it's better through the preamp, although "no one understands why".
Jim Heckman
Jim Heckman