sub add on in bose system

Hi i'm trying to add a subwoofer in a 350z 2008 , but i keep getting a bad signal or bad box ?? Subwoofer is almost new but it sound awfull.  it sound like its out of phase, strange sound a low and hight volume ? I'm using the hight input on the amplifier. I used the signal that was coming from the oem sub amp. So the signal i used is coming from in between the amp and sub, should i used the signal from the radio ?? Also i found that my box was like 1.75 cu feet.  and the spec of the sub are at 1 - 1.2 cu. feet.  Could a to big box cause the wiere noise ?? I would figure that it could cause the weired sound at high volume but even at low volume i get the same sound.  Any help here would be greatly appreciated.  Should i be better off using a hi-lo converter instead of the one on the amp ??  Again i'm looking for your expert advise

Thanks in advance

So I went to the doctor and said I'm in pain, he asked where does it hurt, I answered don't you know you the doctor.

If your runing a single sub, how can it sound out of phase?  disconnect the factory sub, listen for any change.

Plug the box in to some else car and listen, same then box has issue, different then connection/amp.

Wrong size/type of box can cause problems...............but sounding awful doesn't help.

more info

Yes i'm running a single sub .. yeah i no out of phase is not a good comment but when it come to explaining a strange sound its not easy lol . I'm not running the oem sub, it's been removed.  I'm uing the original ''box'' witch was like 4.5 cu feet. so i made a box in a box. wich cut's the size down to 1,75 cu feet.  The sound is like distorted at low and high volume.  Taking the box out of the car is not and option it's built in the car.  But if I remove the sub from the box it sound good at low volume. Soo is the box size really an issue, i know it's important to be in the spec. but could it make such a difference? I will propably try to chop the box size to the recommanded size 1-1.2 cu feet, but i will be a nice project,lol  alway love the extra work..but my friend is getting worried.(it's his car) oh and the box in the box, i really a sealed enclosure, just wanted to clarify (not a banbpass or ported) the sound is really comming for the cone of the woofer and not some rattle in the car,  The sub seem to have alot of motion but not alot of sound.  Running a subsonic filter at 30hz and a low pass filter at 125hz

Waiting for your responce DOCTOR ISPKER !! LOL

Thanks

It is possible to have the input to the amp wired out of phase. Feed signal to only one input and see if it sounds better.

How about cobling together a smaller box to test with.  Is it a small enclouser sub?

Most subs will work reasonable well with a box 20% of recommended size, some will work 50%.  The newer flat subs are very sensitive to box size, to big a box and output drops drastically.

Noises you might be hearing could be mechanical noise from to much movement.

Hi thanks for all you guys help. unfortunetly the guys car is out for the week-end earlier week so i'll only be able to try to figure it out on thursday.  I'll try all your comments out and see what the problem is.  The sub is an Ultimate T3 made by Erikson i think.  I'll try and put it in a sealed enclosure i got a work and see how it performes out of the car .  if it sound good then i'll go and built the smaller box in the car.  A bit of wood here, a few srews there, a a little fiberglass face/grill and it should be good.  I'll also try unpluggin a chanel of the hi-lo converter. maybe i have the polarity of ?? But im using the signal from the OEM sub amp(mono signal) and i tap the two sides (left/right) of the hi-lo converter on the 2 wires + - of the oem subwoofer. 

in the meanwhile if you have any other suggestion i'll try them out, and i will let you know how everything turns out.  Thanks again