Sorry, I don't have any experience with additional subs. However, I think there's a group by on the JL Stealth. Check the group buy section. The JL stealth doesn't take up much room in the trunk and seems to be hidden in the back corner.

Personally, I would rather upgrade the underseat subs and install a smaller amp in the trunk to power them. I like having full use of the trunk for kids' items, hobbies, etc....Also cleaner looking.

 

Ok, if I upgrade the underseat sub, will MS-8 can power that sub? and what sub is the correct one to upgrade?

 

Right now I have the MS-8 powering all speakers including the stock underseat subs. If I were to upgrade the subs, I would get another amp to power the new subs.

I would PM member Technic for your audio questions. He should have more details on what good ones to get.

 

For now, I just want to add a sub + amp to the trunk and using Technic harness to tap on the stock harness for the sub signals then later on I will add the MS-8 as you mention on earlier threat. I did PM Tecchnic and he's back order on the harness for E60 now. Will check back with him in few weeks later.
For the sub, I may go with the JL Audio 12" and an amp. I am trying to find the right sub box for the sub driver All I need for the trunk space is to be able to get the spare tire out if I have flat tire. Event though I have the run flat tires on my car but I will replace the front tires very soon with the NON run flat ( just replaced 2 rear tires with run flat tire cost $1200 toooooo much ) .