Here is a problem I have been trying to deal with for many years. On a few guitars I have owned, I have noticed that there are certain specific spots on the fretboard where the fretted note is lifeless. The guitar is intonated and set up properly, but the resonance of the guitar seems to make this specific note die quickly. It sounds as if the note played at that specific fret rings at an additional octave higher (like an overtone) and then dies quickly. I did a search and found the term "wolf tones". I think this is the phenomenon I am dealing with. Often when I try a guitar at a store, I will immediately check the strings around to 9th to 13th fret to listen for this. I can find it quite quickly and, surprisingly, I find it on quite a few guitars. Many years ago I owned a Roland GR33 guitar synth, and the guitar I attached the GR pickup to had this problem on the G string around the 12 fret. Sure enough, fretting this position caused the synth to sound a note an octave higher. Great tracking on Roland's part! I experimented at the time with clamping metal on the headstock and it cured the problem. It was at that point I began to understand a guitar's personal resonant qualities and what was causing this problem... I hope reader's understand what I am talking about... My fave guitars obviously have none of this, but I have a basic Epi SG that has this problem. I often wonder if there is a trick to solving this problem. How does one add mass to a headstock, or get rid of this problem on a solid body guitar?