For sale: New never used Pioneer subwoofer, that fits inside 13"/14"/15" full size spares, and it's perfect for a Mk.1, if you just want to increase the bass response without getting a subwoofer box, and amplifier. $100, or best offer, including US shipping.
I also have a used Pioneer subwoofer for the spare, that I'd sell for $60 including shipping.
I've run it in my Rabbit GTI for about 5 years, powered by a head unit, using a crossover.
I also have a new passive subwoofer crossover, that would be perfect for this. Connect your hipower speaker output, at least 20 watts RMS, to the crossover, connect your dash, or door speakers to the high pass, 120Hz and up, and run the sub out to the spare sub. $20 including free US shipping.
This crossover is much better than just running bass blockers, because the bass power is not wasted, just diverted to the trunk.
Bass blockers, basically just 2 capacitors to block bass and lets higher Hz pass, good for 4" and smaller speakers, especially if you have a sub. $10 including US shipping.
Discount Dealer of Techtonics and Peloquin, too low to advertise...