Best Sub for under 90ish

Alpine R series is a good sub for a fairly cheap price. especially if you purchase online. remeber buying online voids almost every manufacture warranty that there is.Quad L Handyman services

really there's alot of cheaper subs on like eBay and stuff but like Ketel pointed out, no warranty. Just a suggestion from my part. Go down to bestbuy or circuit city. I know for a fact you can get a Rockford Series One for under $80 (+20 or so for a 3 year warranty) and dispite some peoples opinions (mostly peeps who dont deal with RF or just had a bad experience) their Power Stage Series subs are a great improvment over their older punch and power series in terms of SQ and SPL. As far as eBay goes I've seen everything from MTX to Rockford to JL and on and on for like $60-90. Oh, and just a point, you didn't mention size. 10's are usually cheaper than 12's just as 12's are cheaper than 15's so getting a 10" sub will cost ~30% less than a 15" of the same model."Shiny chrome when used in conjunction with bikini models is particularly effective in inducing brain deficit disorder"

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Monster System on its way.

Check out Kicker Comp VR -they usually retail for $120ish, or online for like $100 shipped. 

If I were you, i would definitely check out ebay for a sub.  You can usually get a good sub for cheaper than retail.  I havent bought a sub yet, but I bought a set of $200 components for around $80.  I would just look there and decide which sub you like more.  If you were to go to a store and purchase a sub, RF series one would be a great sub for your friend.1 plus 1 equals 2

there is many alternatives to ebay also, some better some worse. you may like to try out some of the online companies. i know that ikesound.com and purecaraudio.com have cheap prices although again im not sure about warranties.Quad L Handyman services

JBL GT 10" subs are very impressive for the price, you can find them for $79.95 at local retailers.  If you want something that few people have, you might check into Resonant Engineering.  Their website is at www.respl.com.  Hope that helps!

You can get a Brand new pair of Audiobahn 12's for like $100 shipped.  They get slammed for SQ, but can sound good in the right enclosure(a properly ported box).  It took me several tries, but my 1051Qs(2 10s) finally sound good.  You can get the better series Audiobahns for like $60-90 per sub.