Vintage 1990 Sony X77es as a transport

I just recently traded my pioneer pd65 for a sony x77es. I immediately noticed that the sony had a little more detail and a tighter bass response, but at the expense of sounding a little more analytical. The Pioneer had been in my system consisting of Quicksilver audio gear for over 3 years. I noticed the lack of warmth and naturalness that the Pioneer seemed to always have. Don't get me wrong! I like the sony very much and it's built like a tank. You wont find many players out there today built to the sony standards of the early 90's. There are high quality parts in this machine and if you can still find one, I wouldn't hesitate to purchase one. I just wanted to ask anyone who owns this player or the x777es(1 or 2 yrs newer)if there is a way to upgrade the sony with a dac or certain tweaks. I don't know if you can upgrade the power cord because it is attached. Please let me know if you have any suggestions on this. Chip