Adding subs to a 335i

How should I go about doing this. I have the stock system. My car came with a year of Sirius radio, so I might have premium sound. Can I just connect subs to the stock amplifier? What size and how many subs should I do. Two 12" subs would be good, but I'm thinking two 10" would be a safer bet. Any help greatly appreciated.

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http://www.e90post.com/forums/showthread.php?t=641323

There are three systems world wide that are common one that isn't

stereo = no amp 6 speaker no tweeters also called base system world wide.(Only in the US 2010 and early 2011 )
Hi Fi 8 or 10 speakers depending on the year also called Premium but not the best one.
L7 also called Hk or Top Hi Fi Surround sound with 13 or 11 speakers depending on the year and a 9 channel amp
There is another version of this not common with even more speakers called the personal stereo .
Sirus radio is not a determining factor.

Identifying your system should be your first priority as solutions are different for each one.
You will need another amp if all you want is subs.
Also describing what you want in a system would be good as too many people think subs
are the solution for everything.

I have the hifi system. 4 mids all around. 2 tweeters up front. None in back or behind seats. And 2 underseat subs. I also have a small grey amp with one big cord connecting to it in the trunk.