Haven't tried it. I don't think it would hurt the driver, but my suggestion would be yes, try it with headphones as the right ones might have a better proportion of deep bass. Or better yet, try an external, really efficient 15" cabinet.
If this little amp takes off there will probably be more musical instrument amp designs and we would certainly consider a bass amp. Active loaded parafeed, maybe? That oughta go deep and punchy.
I think it would be fun to do a really fat, rich amp for blues harmonica too. And I have a few ideas for effects.