It is possible to install the mini-sub in the "non-existing" gloves compartment. Best way to access that area is to deinstall the passenger airbag. I took the loooooong way round and cut a whole into the lower dash and used a mesh for better air & sound flow. But I am pretty sure, you can do without that, soundwise. I used strong cable binders to secure the sub in its spot. Additionally, I ran a separate power cable to the battery as it is recommended that way. I am mega convinced of my solution. An invisible installation is absolutely possible, although it is kind of tight. So far my base vibes didn't trigger the airbag to pop...
I am (still) preparing my lower dash (both sides of course) to either upholster that area in cocoa leather or spray paint it in seal gray. That older picture gives a good impression of the spot I used.
There is no way to install a mini sub underneath the seat, I've checked the whole interwebs for available mini-subs...Kennwood KSC-SW11 was the best bet for my attempt.
PIONEER: SPH-BT20 DAB IOS single DIN 4x50 watts (MOSFET-Amp & 31-Band Equalizer via app)
Dash speakers: Infinity REF-4002CFX-2 4" 2-Way Reference X Series Coax speakers (10cm, 105 watts, 35 watts RMS, 3 Ohm)
Door speakers: JBL GTO 6528 2-Way Coax speakers (16,5 cm, 180 watts, 60 watts RMS, 2 Ohm)
Kenwood: KSC-SW11 active subwoofer (1x 150 watts, 4 Ohm)
just for the grins and giggles