I have a ribbon tweeter that I want to drive with the SE84C+.
I am using the marchand 24 dB/octave Linkwitz-Riley active crossover set at 10 KHz.
The tweeter is 8 Ohm and the sensitivity is 95 dB.
My main speakers are Beauhorn Virtuoso III / Lowther DX4.

1. Does the SE84C+ have enough power to drive the ribbon tweeter to match the Lowthers?
2. How does the SE84C+ sound in the upper register?

Hello, and welcome to the forum!  I'll take a stab at answering your questions.

1.  It will be cutting it close, depending on how loud you like to listen.  Sending a high-passed signal will help eek a couple extra dbs out, but the amps will still clip badly past two watts, so use that in your calculations/estimates of the tweets max spl.  Another thing to consider is that the SE84C amps like low impedances, so you may not get full power into that 8 ohm tweet.  
2.  I agree with prior poster's opinion on the sound.  They have crystal clear, hyper-detailed-yet-smooth-as-silk highs, and make excellent tweeter amps.  I did it with pleasing results (after a lot of fiddling around, and that was using horn loaded compression drivers at about 115 db/w, so power was never an issue.)

If you listen at normal levels, it could work very well.  Let us know how it goes, and good luck!