"ebyea" wrote ...
It's for a guitar tone control and I'd like to replace the little 0.1mF
marked 104 light dull brown about 3/8" circular cap with a paper in oil
which I happen to have or a hovland or something of a bit more quality.
Would there be any loss of signal using a different type of capacitor or
is a cap a cap and all that matters is the rating and that the volate
meets or exceeds the requirement?

IMHO, the question is NOT passive vs. active.
The question is whether it is in the signal path or not?

Few people care about transient intermodulation distotion
of a power filter capacitor, for example.

In your case, this is clearly a signal path component.
OTOH, the effort of replacing what is already there
may very well be not worth any potential result.