Yes, I did try preamp-less and preamped options between the DP-777 and Cary 300SE. I prefer the preamped sound, being more ripe, full-bodied, and solid. In comparison the preamp-less sounds a little cardboardy and less 3D, the difference is not as prominent as in some system though, but I consistantly find a good active preamp helpful in various systems (I've bought and sold hundreds on audiogon). Last night I invited a cellist friend to my home and we listened to various cello/string pieces, he confirmed that the preamped sound is closer to a real cello.
George, you are right the Cary is a low-gain amp and it certainly helps to reduce system noise in this case! But a good active preamp is the soul of a system and many people can't live without it. Nelson Pass has a point, but other designers at Conrad Johnson, Audio Research, Supratek, etc are not dumb either.
I should mention that Audio Mirror's T-61 tube preamp is a tiny little wonder for ~$1K. It replaced a trusty CJ Premier 14 tube preamp as well as an Audia Flight Pre SS preamp in my system. It's innovative in using LED (instead of a resistor) to ground the tube circuitry.