Speaker recommendations

PSB Synchrony Two (private sales).

An amp will seldom overpower a speaker. In fact, blown speakers or poor speaker performance is usually the result of underpowering them. More power is always a good thing.

If the sound is not as bright as you would like, then it is the speaker that is not to your liking. Too much power from the amp would have nothing to do with it. But there is probably something else at play here causing the treble to not sound right. Most likely its your room, the furnishings in your room and the placement of the speakers relative to your listening position. I suggest you experiment with all of these factors first before you rush out to get new speakers.

Good answer, paraneer. I'll add that the type of speaker cable you are using will affect sonics. And the impedance of the speaker cable plays a part in the sonics of amp and speaker.
Also, a stranded copper conductor will typically present smoother highs than a solid core speaker cable such as Audioquest. When silver is used as a conductor, it will bring out the higher frequencies.

What speakers cables are you currently using? You may need a cable with a broader bandwidth to get more highs through your speakers.

i guess I'm not that dissatisfied with my any specific characteristics of my present Triangle speakers but figured maybe incorrectly that if I upgrade to better speakers that sound quality would generally improve. So the basic point of this line of inquiry is what speakers do you folks know about that would provide a somewhat noticeable (I know that is subjective) improvement in overall sound  quality, particularly in the mid and high range for less than $2,000 (incl. used) for the pair. My listening room is about 12' x 16'

Investigate PSB synchrony’s. Sensitivity is a cinch for the 375BEE as well as there 4 ohms impedance, to add to that power handling is about as ideal. Safe bet they would be a very nice step to you and work excellent. PSB and NAD also happen to have great synergy together if lots of detail and loads of musicality is your thing. I take reviews with a grain of salt, there is no short of positive buzz out there. I have also heard the combination along with plenty of other PSB/NAD partnerships that I will vouch for in confidence. Mids and highs noted, those are just a couple of there strong points. 

http://www.psbspeakers.com/articles/TAS-Synchrony-Review
http://www.psbspeakers.com/products/synchrony/Synchrony-Two#heading-downloads
http://www.psbspeakers.com/content/110620153653-PSB_Synchrony_Series_English_Manual.pdf

hbdelrio1, you should have a look at (i.e. at least consider) the GoldenEar Technology line. Something like a Triton 3+ I believe would fit in your price range.