Having had the Adcom GFP-750, it's good in passive mode but as an active preamp it leaves much to be desired. I bettered it in one of my systems with a BAT VK3i.
You've got a very, very nice system going and a tube preamp would take it to yet another level. I'm a BAT fan, so I am biased, but even the 3i would be better than the Adcom, and cost very little more to get to.
Best to you!
beemer