Seeking speaker purchase advice


Hello,
I am presently running a pair of Tannoy Revolution XT8F, with my Hegel H20 power amplifier (200 Wpc into 8 Ohm, 350 WPc into 4 Ohm) and RME ADI-2 DAC/preamp. Listening to classic rock and classical music CDs and HD tracks (24/192). The Tannoys are very good speakers, but I am looking to upgrade to, possibly, a ribbon tweeter speaker.  Evidently, I am open to any other recommendations. My budget is limited to $4,000 though. 
My listening room as 13' x 21' x 8 ft high, with rugs and some upholstered furniture. 
Your advice would be highly appreciated. 
raduval
What specifically are you looking to improve upon?  Just saying you want to move to a ribbon tweeter doesn't say much.  I can't believe the only improvement you're looking for is only in the tweeter.  Tell us what improvements you'd really like to have overall.  $4000 can get you some damn fine speakers, especially I'd you're open to buying used. 
Ascend Acoustics Sierra Tower with RAAL upgrade comes out to around $3000 shipped, more for custom finishes.