dub not dub

how far do th bounds of dub stretch? i think there's a case to be made that any of these fit:

bark psychosis - absent friend

fishmans - nightcruising

burial - ghost hardware (note: don't listen to it on laptop speakers; you'll miss half the song)

tortoise - galapagos

sub oslo - celestial penetration
so what is it?
the spring reverb?
the tape echo?
the upstroke?
bass?