Best bookshelf w/ ribbon tweeter under $2000

If you can find them used, consider a pair of VMPS 626 JR's.

The Aon3 does not use a ribbon, but an Air Motion Transformer for the tweeter. They are both subsets of "planar" transducers. However, with AMTs, the drive current is carried via a conductor that is imprinted/glued to the diaphragm whereas with a true ribbon, the drive current flows directly in the ribbon itself.

Johnnyb53 points out that the AMT on the Aon3 has superior transient capabilities than most domes. Ribbons have even lower mass than AMTs, and when done well, are unmatched by anything other than plasma tweeters.

As for the 8 Ohm/90dB sensitivity rating coupled with a 38Hz cutoff in an enclosure of that size, take that with a grain of salt. A typical 7" woofer that can reach the upper 30s with 90dB sensitivity will require a cabinet over 1 cubic foot which is quite a bit larger than that of the Aon3. Either the sensitivity is lower than they specify, or the actual cutoff frequency is much higher.

From the measurements below, it's not a bad little monitor that offers decent linear response and fairly low distortion.

http://www.soundstagenetwork.com/index.php?option=com_content&view=article&id=762%3Anrc-measurements-goldenear-technology-aon-3-loudspeakers&catid=77%3Aloudspeaker-measurements&Itemid=18

http://www.skiingninja.com/GR-N2X-Ready-to-Play-p/grn2xfull.htm

Technically, a planar tweeter. GR midwoofer has a few dB of BSC without losing efficiency. Sealed, so bass is tight but at the cost of extension. Although I'm not familiar with this particular speaker, I can recommend the components.