Good amp for Thiel 3.2's ?

I have a pair of Thiel 3.2's that I really like but that may need a better amp. I am currently driving them with a NAD 218 THX which was only $900.00 new and seemed like a good deal. One of the problems in selecting an amp for these speakers is that the bass is really muddy with less that 150 watts (Theil recommends 200 watts per channel). Before I got the NAD (250 wpc) I was trying to drive them off a 85wpc integrated and it couldn't drive them at all. The NAD drives them fine but when I tried to hook up a Mark Levinson 39 direct to the amp (no good preamp yet) the overall effect was much to bright and fatigueing. These speakers are very open and airy which is why I like them but you have to be careful what you feed them.

I guess my other option would be to buy a nice tube preamp to tone down the system but since this is known to be only a low to mid grade amp I was hoping to start by replacing it.

Please let me know what would be a good match for these power hungry open and slightly bright speakers.

My budget is around $1500 to $2000 used.

Thanks,

Montana Rane