Growl to me is the bass on many of the Boston songs. I describe it as when each note oscillates. I've never heard a live bass sounds like that. There is another thread on the tricks of the recording trade.
I do know during the David Sykes Boston-era (late 80's-early 90's),he (very) surprisingly was playing a Peavey Dyna Bass 5-string. That's not a typo, either!! A Peavey Dyna Bass 5!
