I have a couple of underseat sub boxes for sale:
First up:
Brand new, never used Aussie 8" box.
This was $210 shipped & almost a full month wait time. Selling for $200 shipped
Next:
Used, Aussie 10" box. This was a test box for multiple different subs. $65 shipped
Last:
8" 3D printed square...
Very true on the large crossovers. When I initially did my speakers, I did the Kicker KSS690 running off of the stock B&O. I had some of the small inline kicker crossovers left over from a previous install & used those for the tweeters.
Do not skip the crossover for the tweeters. The stock tweeters, be it B&O or non, have a capacitor built in to the tweeter itself to limit the lower frequencies.