Best Sub Under $175

Alpine type-R

Eclipse Aluminum

Infinity

Check out this, MTX top-of-the-line from last year (the 18" competition subwoofer being the only one better):  http://www.cardomain.com/item/MTXT81044A

If he has the space for a single 12 that will slam look into this.  http://www.cardomain.com/item/JBLP1220

Neither of these subs are a joke, you will need plenty of amplifier power to make them sound their best.  They both say 400w and mean it, ported or sealed they take POWER, I would not bother trying to run either of these below 300w from a good amp!  Not Boss, not Pyle, Pyramid or other pawnshop brands. If you have the amp already and want to stick with a sub 300w power range try JBL GT series, type R alpine, MTX 7500 or 5500, JL W3v2, there are others.

I would personally get nothing less then 400w rms and feel comfortable saying you should get MORE power (over 400w) rather then less.

Ask if you want amplifier recommendations.

This distinction between 300 and 400w and the assertion about sounding their best is, in my opinion, false and misleading.  See my long post here:  https://www.the12volt.com/installbay/forum_posts.asp?tid=48102&PN=1

This is the kind of mis-information that commissioned sales people and amplifier manufacturers love.  "It won't sound the best unless you buy my more expensive product."  Please.  Stop the madness. 

Heavy, I'm not trying to pick on you.  It isn't your fault we're bombarded with screams of MORE POWER from every direction.

No, I know your not trying to pick on me.

I've always said if your within 75% of the woofers rms power rating you have little to gain by adding that extra 25%, or more.  So, if he has a amp rated at a believable 300w, that would be good enough in my opinion.  Or, if he has a amp and wants to keep it, and its rated at say 150w, I would look into a different sub (when you drop bellow 50% I say look to a different woofer).  However, if he does not have a subamp at all, I say: get a amp that is at or slightly over the rated rms of your sub, headroom is all, and maybe future proofing your setup.  Ive never regretted choosing a amp with a bit of headroom.

I understand there is little to be gained when you get close to the rms rating of a sub.  I did not mean to imply that the sub would perform noticeably better with 500w vs. 300w, cause it mostly likely would not (not for most users, you would just increase your chance of frying up a subwoofer).  Double or triple the power and you will gain little, what I was talking about was headroom and future-proofing the system. 

Typically, you don’t pay that much more for a substantial jump in rms power 50-70 dollars will get  you the next amp in the line.  But if someone was trying to tell you that you cannot run a 400w sub off a 300w amp and have it sound good and you need the 400w upgrade amp for an extra $200, tell em to shove it.

Anyhow, no worries, thats my rational, more or less.  These issues can be complicated and I was trying to make a statement without complicating the issue to much.  Often we comment on issues that are more complicated then we can express in a reasonable lenght, so we make a quick statement, not meant to be read into very deeply.  I think I may have come down on the MORE POWER side a little hard, opps.