Depends how much time and money you want to put in, but some people have built a sub box to fit under the luggage shelf. If you dont mind losing the spare tire, I bet a 12" would work. Keeping the spare, maybe a shallow 10", with the amp located in the quarter panel.

 

Actuallyy, I have seen several subs with built in amps that are supposed to fit under the seat. Never tried them but their ads look very impressive. And since they are under you, the bass should be good. Crutchfield has a few pages of them.

If you go this route, let me know how they are as I am considering one for my Jeep.

 

None of the Crutchfield will fit the HHR.

 

i am sure that i saw an old post about this. it was a replacement/upgrade sub that fit in the oem pioneer sub mount.

 

The depth of the factory sub box is roughly 4 inches (I can find out exactly if you need me to). You can put a shallow mount 10 inch sub in the factory location but not in the factory box. You would need to make a new box using fiberglass to make the 10 work (but it will work). What I did was use the factory box and put in the best 8 incher for it (low Q). I did not have to worry about speaker depth because I removed the factory grill and mounted it with the magnet facing out (not very stealthy but very functional). I also wrapped the factory enclosure with sound deadening material. My bass is loud and deep. When I play my kids hip hop music it will shake your eyes in your head (seriously). For a little while I was running it with the factory amp before I finished my system and it sounded light years better than the factory sub. Oh yeah, the factory amp can run a 2 ohm sub so make sure you get a 2 ohm sub (or a DVC 4 ohm sub) to fully take advantage of the factory amplifier. Now I mention this because I could have mounted it normal and made a 3/4" - 1" baffle ring to make the depth correct. You would still have to remove the factory grill but you could buy or make something that would look factory and still clear you subwoofer. I may be doing this soon myself as I have only recently found out that I cant use my false foor as a shelf now because the magnet just barely interferes with it! Oops I'm also contemplating switching to an SPL setup (no hiding that )Here are pics of what I did:

Hidden underneath the floor is the EQ and Amp:


With the floor in all you can see is the sub magnet. If you mount it normal and use a ring to make up for the depth difference you could use a nice 10" grill to cover it and still make it look factory to keep wandering eyes from wanting to take what you worked so very hard for

 

You have just inspired me! One question, with the EQ and amp setup, is the spare tire still in there? I have the worst luck on the planet, combined with New Jersey roads; so leaving the house without a spare is not an option.

 

Yup, it's still in there and easily accessible. If you look at the grey colored carpet its somewhat recessed into the plastic tray above the spare tire. There is plenty of dept there for almost anything (approx 3"). I brought all the wires and cables in through the back side (closest to the rear seat) so I could lift it to get to the spare. Only difference is I had to ditch the big plastic nut that held it down bacause I wouldnt be able to get to it with the amp and EQ in there. Like I said, make a baffle to extend the the depth of your sub and buy a cheapy 10" subwoofer grill to neatly finish off the hole. I keep saying 10" grill because the hole is already to big for an 8" grill. Or you could easily make your own grill with some MDF and grill cloth. I like the wire mesh grills myself (look more professional to me and you can paint them to match). Youll find the hardest part is getting that damn rear quarterpanel off and on (it's a pain). Once in there youll see what room you have. I even mounted an epicenter type bass module to the box where the factory amp used to be (I can adjust it if needed through the taillight access panel). If I had the money Sony makes these tiny little amps (one is 4 ch and the other is mono) that could both fit in there behind the panel and everything would be 100% hidden and all the little factory pockets and trays would be 100% in tack and useful.