If the speaker under discussion is the Thiel CS2.2 then the frequency response graph from the Sterophile review done in 1993 may partially explain where the brightness is coming from.
This graph shows the averaged frequency response in John Atkinson's listening room at the listening position.
John mentions that there is "prominence" in the mid-range's passband which may make the speaker fussy if partnered with amplifiers which themselves are bright sounding. I would say that the 3dB "hump" centered at about 1.8kHz is the heart of brightness you hear. The quasi-anechoic response curve clearly shows this 3dB hump at 1.8kHz as well as the somewhat ragged response contributed by the tweeter in the upper octaves, which may or may not be part of the problem. See image below.
If the speaker is bright sounding and I were using these speakers in a system with a computer to play back music files,
I would use jRiver's parametric equalizer capability to flatten out that mid-range hump and fix the speakers response aberrations.
A rebuild of the crossover with new caps won't do anything to the hump in the mid-range and it also won't alter the tweeter's inherent response curve.
Here is a cut-away image of the Thiel CS2.2 showing some of the crossovers complexity.
Scotty
