My own experience was almost exclusively singing-with-accompaniment (acoustic guitar).

When I first ventured out on the street (my first *day* only), I was too scared to sing -- suffering "stage fright" without even being on a stage -- and had no success attracting any contributions at all with my low-volume fingerpicking. Only after I summoned up the gumption to open my throat and sing (and to play louder, albeit more simply, at the same time) did I find any success.

I've seen plenty of successful instrumental-only busking acts, but very few guitar soloists among them. The instrumentalists who do best seem to be those who appear in groups and/or who play "interesting" instruments (e.g., harp, hammered dulcimer; even a fiddle is "exotic' enough -- or maybe just loud enough -- to attract a little extra interest).

Unaccompanied singing might just be unusual enough to command a lot of attention. I'm sure you'd have to be *very good* and fairly loud)to pull it off.

Classical players, solo or in trios, quartets, etc., seem to do very well just about everywhere. More power to 'em, but it's not very helpful to advise a folkie to go out and "Learn classical music."