Help needed for aftermarket sub

[MerseyRayUk]

Hi,
I've read a few of the posts on here but can't find the answers I'm looking for.

My E88 had the very basic speakers (base system?)
So Ive just purchased and installed the HK front door speakers and tweeters. These are sounding better, but I'm after making improvements.

I know the front speakers and subs are currently wired in parallel, so I want to separate the under seat subs from the front channel so that I can use the HP filter on the door speakers and tweeters, and put the subs on their own amp which will probably end up under the boot carpet next to the battery.

The things I need to find the answers to are:

1) Does my E88 with base system already have any amp anywhere?
2) Is there any type of filter already in line with the front speakers/subs
3) My HU has DAB+ so what's the best way to get a DAB aerial? Can the existing small aerial be replaced with a DAB compatible one, or would I need a fin on the boot.

Thanks in advance.