the 600 watt 15 inch isp sub is one of the best things i ever bought for my rig.
juggs hears me swear by it all the time. can't suggest this piece of gear enough.
a lot of people have a lot of misconceptions about it. until you try one, you won't understand. the only way to describe it is "full 3-d type sound." yeah, it adds bottom end. but what i think it really does is tighten up your bass since it eliminates the ultra low end from flexing your 12 inch speakers. that's one of the things that causes a "muddy" or "flabby" guitar sound.
yes, guitar is a midrange instrument. yes, bass is a bottom frequency instrument. but both bass and guitar use each others frequencies all the time. it's all about HOW you share those frequencies.
would i record with one? no. a good 4x12 cab mic'ed properly and correct e.q. at the board is all most of us need. but live, i would never be without my isp sub. once you try one you will understand.