Mark, as an oldtime fiddler for the past 33 years, I have to disagree with you about bass not being a part of it. Having a bass play with you really sets the music off and frees everyone up. It gives it that driving sound we all look for. Bass is always welcome in any oldtime band or jam session. Jews harp? I've never played with one! Mandolin is iffy. It takes someone with great finesse and feel to blend in well with oldtime, but it can be done. What is oldtime music? You know it when you hear it. And once you've heard it, then you'll know. Until then, no verbal description will suffice.
Al