Slimline active sub in A4 (B5) boot - loud enough to be heard?

I have a lovely boxed and sealed 10" Pioneer sub in my crappy daily - a Toyota Corolla hatchback and it sounds great.

I want to upgrade the Audio in the wifes A4 saloon but I'm struggling a bit as the boot gets used all the time for boxy items and having a big sub that needs taking in and out is not a good option.

So.... I have considered an active slimline sub (or two if needed) which could be secured to the rear seat or elsewhere within the boot but are they going to be heard?

A neighbout has A Rainbow Intelli active sub:
Rainbow Intelli Sub 8 AFE 20cm Underseat Amplified Subwoofer

It is in the boot of his 5 series estate and it works well, but the rear load area is not sealed off from the cabin like it is on my A4 which has the solid metal rear seat bulkhead. Being Rainbow it is great quality but also expensive.

There may be the option of me buying it from him, but for the same money I could buy a pair of equivalents from a lesser brand like Fusion/Vibe etc?

Has anyone tried a smaller sub in a B5 boot?

Stu

The sub was installed by the supplier in a side pocket in the rear load area and is able to vent through a large grill. It does work well and although it obviously a compromise it really adds something to the sound.

The front end is streamed music through Parrot and Becker, Focal amp and components, so all good quality kit which is why he opted for something from Rainbow.

Maybe the 5 series doesn't lend itself to much under seat room, I don't know.

But, what I'm worried about is if the sealed boot area will provide too much resistance for the sound to be heard in my A4?

Perhaps suspending one underneath one of the rear speaker holes if I remove the speaker would assist? I could do with a bit of an idea before I spend any cash.

If it's not going to be much good, I may as well fit a pair of the Bose 'subs' from the Bose equipped A4 which go for peanuts on eBay and just amp them with a low pass filter?

Stu.