OK, I'm chiming in a bit late on this thread (two years, in fact!) but I have a couple very relevant points to make.
To help explain the contradictory comments posted in this thread about the de Capo's high frequency performance, I say this:
1. When using an amplifier with high output impedance (every SET I know of), a speaker's frequency response will be similar to the speaker's impedance curve (have a look at Stereophile's measurements of the de Capo in the December 2003 issue). It is for this reason that I regularly disregard a person's opinion of any speaker when auditioned with a SET amp, unless that speaker in fact has a relatively flat impedance curve. A speaker that does not, when driven by a SET amp, will sound drastically different than when driven by an amp of a different design, not so much because the amplifiers simply sound different, but because the high output impedance of the amp makes the overall sound of the combination of amp and speaker dependent upon the impedance curve of the speaker.
2. The de Capo is very sensitive to both toe-in and rake angle (the forward tilt of the front baffle). Again, look at the Stereophile measurements. The de Capo has a broad 4 dB peak that stretches from 5 kHz to 20 kHz and peaks at 12 kHz. Knowing this, Reference 3A recommends experimentation with both toe-in and rake adjustments.
Hence, when driven by a SET amp and auditioned on-axis with the listening position, the de Capo will probably sound bright.
Perhaps this helps explain some of the contradictory comments made in this thread.
Cheers