Fingertips, Nuts & Capos Sterner TM

"I hate playing with capo. The capo kills the tone of my instrument and I loose the nice ringing of the open strings."
Of course it's a matter of personal taste, but in my opinion a statement like the above indicates an unbalanced instrument. No matter if the strings are stopped by the fingers, the nut or by the capo they should have the same tonal quality. If the open strings sounds brighter and have more sustain than the fretted or capoed strings, the nut is probably too hard.
In one end the strings are stopped by the bridge, but in the other end the strings are stopped in three different ways: 1) by the nut, 2) by the fingers and 3) by the capo.
The overall sound of the instrument is set by the bridge. A thin, hard bridge gives you a bright sound. A thick, soft bridge will produce a mellow sound.

In the other end of the strings the sound should be determined by the fingers. Since most notes are fretted by the fingers and the fretted sound is difficult to alter, the fretted sound should determine the sound produced by the nut and by the capo.
    The nut determines the sound of the open strings. Of the three ways of stopping the strings in this end, the tone produced by the nut is the easiest to controll by choise of material. As with the bridge, hard materials gives a brighter tone and soft materials gives a more mellow tone. Often the nut is too hard, giving a bright tone with lots of sustain that differs from the more mellow sound produced by the fingers. If this is the case - try a softer nut.
    The capo should cause no problem. The soft pad produces a tone similar to the fingertips. A good capo used in the right way, will not alter the tone on a balanced instrument.