The sony PFR-V1's are a good example of tubular headphones. One advantage for using tubes to transport sound is in the ability to reduce cupping resonnances to virtually nothing.
Sony has apparently coined the term 'Personal Field Speakers'.
Apparently they sound great, however they look atrocious!!!
Transporting bass is most suitable for tubes, a larger wavelength theoretically should maintain sound quality.
Here's a review
http://reviews.cnet.co.uk/accessories/0,39100116,49296438,00.htm
[url=http://crave.cnet.co.uk/cnetuk/crave/digitalmusic/0,39029432,49296360,00.htm]