Another vote for the tube pre first. I think you'll be extremly happy with the upgrade. I also went the tube amp route, later, and was even happier. Obviously, the care and feeding of a tube amp is much higher - something to think about. I love my audible illusions M3 but too much gain can be a problem in some systems. I've heard and liked the VTL 2.5 and have seen these used for about $900-1000.
Good Luck
Good Luck