Some speakers need it.Unless you have something so pure you don't want to screw it up (I was thinking a narrow range system like 47 Labs Essence I heard once).Some folks love their Quads or Maggies but their strengths mean seamless integration is tough and more has been written about those two speakers than any.Sure if you want HT 'Boom In Room" try a Velodyne 1812 (JL is a good large sub and price pint wise better than ginant Velodyne).But for music is you want to hear below low note on an electric bass (40Hrz) and have quality then go for a multi driver (usually 3) not larger than 6" or 8maybe 10" as they as rule are faster and give better pitch definition.Some names like REL,MJ Acoustics (employee spin off of REL).Electronics help so adding EQ SMS-1 helps or like Thiel where you don't have 18" driver with enormous magnets.If you have a nice two way which just can't get down their a sub can add but with best systems it's a trade off it's just that some people want to hear and feel Organ music or such down to low 20's.I have heard some great horns with them.One that is a single driver with fast light pitch accurate footprint is Magellan TBI which I know one guy with Avantgarde swapped larger much more expensive subs made by Avantgarde themselves.He loved them.One other 'Gonner I read made a Fostex and ribbon dipole and put a number of subs into his floor joists and linked them all up with a DEQX electronic crossover to control all drivers.I wished I heard that.Sometimes they work.If someone thinks they might have a hybrid two way of quality and wants HT in same Martin Logan Depth is a 3 8" multi driver that with an EQ is great but not as refined as REL which has been 2 channel fave for years at a sane price.Now they have revamped their line and to get original designs you need to buy two top end Stentor and Stadium.This is from what I have heard and read.So they can work but if you can be satisfied with what you can get out of two boxes all things being equal that is better.
Chazz
Chazz