Bluegrass: Highly stylized performance music evolved from traditional string-band
sources but modified to exhibit technical proficiency on guitars, banjos, fiddles and bass.
Themes include contemporary songs ala country music (Nashville style) or religious themes.
Harmonies feature quartet or duo singing ala gospel groups.

Old Time: String-band dance music. The purpose originally to accompany dancing,
sets, squares etc.

Individual players I don't believe fall into the category. I would call them traditional folk singers such as Clarence Ashley, Doc Boggs, Buell Kazee, Roscoe Holcomb, Doc Watson, Jean Ritchie etc.

If you separate the old time string band from the dancers, it becomes kinda' repetitive.
The energy comes from accompanying dancers.

Stringsinger