Err.. Don't think you can put a capacitor in series with the volume pot, I'm sure someone better than me will be able to answer that, but it does sound like you'd just be better off with a tone control to be honest.
Thinks about it for a bit
You could get something like this double pot* and set one up as a volume and the other as a tone so you're turning both down at the same time, choose the size of the cap to effect the amount of top roll off.
* https://www.unofficialwarmoth.com/index.php?topic=34302.0