The SQ thing really depends.. Most of the reason you can hear subs behind you is one of 2 things.. 1. if anything behind you rattles, you'll hear the rattling behind you, drawing your attention backwards.. SQ cars shouldn't rattle.. 2. sub distortion in the higher bass notes are easy to localize behind you as well, SQ subs shouldn't distort on higher bass notes.. Trust me, if the car is deadened and it's a true SQ sub back there, most smaller passenger cars dont localize until close to 200hz, even most SUV's can do 80-120hz no issues. If you play a 120hz tone in my eldorado, even with ONLY the subs playing it sounds like it's on the dash, freaked me out first time I heard it. lol. Anyways another advantage is you can let your smaller less capable front speakers use less excursion which GREATLY lowers their distortion and stops them from rattling the doors.
Really the sub can only play to 60hz thing is a myth.. I used to buy into it too until I had drivers that could do it and a car that didnt' rattle.. You get alot more ouput overall from letting your subs play higher, more midbass attack, less midrange distortion, just louder and cleaner music.
Also, here is my 21, should be a beast down to 30hz or so.
B&C SPEAKERS