Every sub is going to be different depending on the enclosure. Some subs need more space than many people will provide it. Some subs don't work well ported or in a bandpass box. Some just don't work in sealed. Each sub is different. As far as his assessment into which subs would be better for SQ. Well, I think everyone else covered it. Half the people out there that claim to be audiophiles really aren't when it comes down to it. I used to think I kinda was many years ago, but found out I was so wrong. SQ subs have a wide definition, as everyone's idea of SQ is different. Are you wanting something to sound nice as you cruise down the road? Something that isn't overpowering the rest of the music? Or are you looking for something to accurately produce a wide range of bass frequencies? The way a SQ sub is tuned is much lower than one for cruising around the road. I have 2 ported 15's that are set to cover very low frequencies to the point where you can barely hear them and in a few cases just feel them. If you are truely looking for SQ subs then there are more out there than many people think. I know people that have some of the best cars SQ related in the world and have used the following brands.
Kicker DVC (old school ones about 7 yrs ago), Focal (my choice), Arc Audio, Rainbow, Treo and Diamond.
Each of those brands can sound like crap if they aren't given the correct amount of power and right size enclosure. There is actually an art to the design of boxes to minimize port noise and cause certain frequencies to be more pronounced than others. Adjusting box size will adjust sub response and also if you have port/bandpass/sealed box. If you want to find a good sub for your purpose, find a shop that does more custom boxes and ask if they have a few examples of customers who will let you listen to their system. That's my 2 cents.