When I read your example “fox on the run” for some reason, the Sweet version popped into my head (I…I…I don’t want to know your name, 'cause you no longer look the same)… I thought “no way.” I did a little youtube research and came up with the manfred mann version, followed by the Country Gentleman version and it made sense. Picture me trying to place the sweet version into bluegrass… it wasn’t working for me.
That’s a fine question on how we define BG. I think it’s very fuzzy around the edges. Folk, old time, traditional, blues and jazz get mixed in there and that’s part of the fun. I grew up around BG, and I listened to it for years before playing it. I guess for me the most recognizable feature that yells bluegrass is the composition of the band (GTR, mando, banjo, upright, fiddle and dobro). Of course one can grab that set up and play classical or whatever. So I guess a big part depends on the style of something as well. Take an irish reel… one could do it traditional Irish, or straight BG, or somewhere in between, so i don’t think it is 100% based on tradition (that would argue towards arrangement in part). I guess Bill Monroe’s bluegrass boys defined BG years ago and it has been pushed an prodded in different directions since then. That last comment lead me to go search to see what Bill said about it… this is from wiki (so it has to be true): bluegrass is “Scottish bagpipes and ole-time fiddlin’. It’s Methodist and Holiness and Baptist. It’s blues and jazz, and it has a high lonesome sound.” I think that last part is the most often quoted part… that “high lonesome sound.” Much of what I consider BG doesn’t have that high lonesome sound (Krauss). So in a nutshell, I don’t have a clue. Perhaps such questions are what lead someone (attributed to several including Duke Ellington) to say, “There are only two types of music… good music and bad music.”