@enerlevel Sir thankyou for the comment 
although i have changed the subs and the 4 channel amplifier.
@ faisi thank you sir As it goes:
I have both SPL as well as SQ setup installed.
07 Civic as we have seen its a SPL setup, where as 09 Corolla (the other car) has a SQ setup i-e H.U., Front Stage, 4-channel amp and a sealed sub. no rear fill.(will post vids n pics soon). Meaning i might have a clear idea about how a SQ and a SPL setup sounds.
Now, for the sake of discussion, since we are all here to learn from each other and help each other out according to our experience.
What we have here is a wrong perception about a SQ setup and same goes for a SQ sub.
What i have learned over the years about a SQ setup has been beautifully put by "Navin" from team BHP forums: "A good sub's presense is not heard but it's absense is felt." He said it all in just one sentence.
Here is a link which clearly and perfectly describes about a SQ sub.
SQ subwoofer - what exactly is it? - Team-BHP
Here is the best part: By:hydrashok:
"Well, IMHO an SQ sub:
1) Reproduces Bass frequencies exactly as they are meant to sound (no boosting/cutting). Also, reproduces ALL bass frequencies faithfully, and not just a few bands. And there will be no uh, "hangovers" or "ghosts" of the previous bass notes lingering on to colour the bass note being played currently at any point of time.
2) Instruments (like the bass guitar) sound as close as they would sound, had t hey been played live.
3) Blends in with the rest of the speakers seamlessly.
4) Concentrates on reproducing the finest of details, rather than raw volume of bass.
5) Can reproduce even the quickest of transient notes.
6) In order to do all of the above, the sub will play fast and tight. And IMHO the sub will normally need to be a low-excursion design to do this.
The best SQ Sub I've heard is the IDQ V2. Haven't heard the IDMax or the IDQ v3. I haven't listened to a whole lot of other subs either. But IMHO, the IDQ satisfies all of the above points I've mentioned.
EDIT: The enclosure in which the sub is placed will also have a lot to do with how "SQ" the sub will sound -- even in the case of a sealed enclosure. So the SQ "sub" is actually the SQ sub-driver + the SQ enclosure."
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as far as i have experienced with 121spl, 3004spl and 3001, none can do the stuff mentioned above.
More over how many low notes does a SQ song have, i guess none.... So why install a sub which is only capable of hitting low notes like 3001 for a SQ setup meant to play SQ tracks? Low notes are in tracks like Hip hops, raps etc.
Anyways have an open mind about this discussion.
I agree with the above mentioned stuff, your opinion may differ.
@ faisi: located a F16D, interested? but it's not originally made by Solid Audio....??? 
