Reaplce Thiel 3.6's Tweeter

I hope you realize that Thiel's designs are time and phase coherent systems. Thiel goes to great effort to design cross-overs that compensate for any driver irregularities. My suggestion would be to leave well enough alone and consider other options. Perhaps different amplification.

Amp choices can be very personal. Thiel doesn't go as far as to make actual recommendations, how ever they suggest that they have especially enjoyed results with: Ayre, Boulder, Classe', Conrad-Johnson, Krell, Levinson, Pass, Theta, and VTL. Their web site also offers less expensive alternatives. IMHO, Thiel's demand quality amplification. The 3.6's will require an amp to provide plenty of power into low impedances. Be especially carefull when opting for tubes as the difficult impedance load of the 3.6's will require very powerfull tube amplification. Of course power requirements will ultimately depend on your room and desired volume. The Thiels' also have wide dispersion. Perhaps room treatment might help. Try to avoid too much or for that matter any toe in. Good luck.

Of course the easiest way to resolve this issue might be with some equalization.