Installed one of the Kenwoods under the driver's seat at the weekend. Huge difference to the sound and well worth while for not much money.
Bought the adapter too but not sure really worthwhile other than to make sure no changes or cutting into the original car's system. It's a tight squeeze to get it in. If I was doing it again, think I'd just tap into the original speaker cables TBH. But does allow you to get all the wiring done and tested on the bench rather than under the dash, so then it's just plug and play.
You need to cut into the speaker cables on the adapter anyway - I just used T-taps.
Hardest part is disconnecting the lock on the glovebox to release it!
Kenwood KSC-SW11 Compact Active Amplified Under Seat Powered Subwoofer 150W - £99.74 on ebay
Connects2 CT10SK03 T-Harness - £19.99 on ebay
Gebildet 12V 24V Standard Add-a-Circuit Fuse - £7.99 on amazon (for 4, used 2)
https://www.amazon.co.uk/gp/product/B07KWK9FYM/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&psc=1
T-taps £6.90 on amazon
Couple of hours to wire and fit.
Anyway, thanks for the pointers on the thread here, helped a lot, and well worthwhile.