Capo position and tone

[re17]

This might seem an almost trivial thing to ask about but I'd like to see what other people think. The usual wisdom is that if you use a capo, you can expect a certain amount of deviation from normal pitch - notes go up a shade - and also a somewhat more subdued tone. But is the effect more pronounced on some frets than others?

On my current acoustic, while I hear little difference in tone if I play capo 1-4, with the capo on frets 5-7 (which happens to suit quite a few songs I strum and sing along to), I find the guitar sounding much mellower, which is actually not to my taste. Assuming my ears aren't playing tricks on me (they might well be), what causes this duller tone on these capo positions particularly? I have a feeling it's something to do with the fact that the strings are at their maximum height above the frets here, so capo-ing in this area means you lose that latitude for the strings to vibrate. Is there anything I can do to improve treble when playing a capo in this central zone of the neck? I'm already using the brightest-sounding picks I can find.

Cheers.

Richard