BOSE AMP question please

[Designo_E320]

Hello Guys,
I have the bose system in my 2000 E320 and I have 3 questions/issues to ask about please:
1. Can I just replace my speakers without any worry as long as the sizes are the same as I am planning on replacing ALL speakers EXCEPT the factory tweeters in the front? Sop can I just disconnect the original speakers and replace them with JBLs?
2. While I was looking through the trunk, I noticed that there were only 2 speakers (6.75") hooked up (BOSE) and there are still 2 holes in the middle that are not being used. I was planning on putting 2 free air kickers subwoofers in there which will also be the 6.75" ONLY as to get a perfect setting without making boxes, cutting etc.... Do wires for this already exist and are they plug and play? If not, then can the BOSE AMP already in there push these kickers that are only 6.75" and can I run some wires from the AMP (does the amp have any extra channels that can be utilized? If so please explain.
3. Can someone please tell me where the AMP is located?
I thank you guys very much!