for inspiration my upgrade (Slovak language)
KOMPLETKA HYUNDAI I30 JBL STP DLS GLADEN
Thanks for that (great photos too!) Here is an english translation of the text in that link.
HYUNDAI I30 JBL STP DLS GLADEN COMPLEX
Written on: January 19, 2018
The reproduction of the sound in this autic did not suit the owner and our task was to correct it. The choice of speakers fell on DLS MB6.2 to the front and the DLS M526 on the rear door. Of course, the doors were well-drained by the STP gold and STP Silver and the Gladen . This is the best for both reproduction and comfort for riding. As the serial radio does not deliver and does not deliver the required performance, we put the 5-channel JBL CLUB 4505 amplifier into the double-deck which provides enough performance, OEM radio compatibility and small size dimensions for easier integration into the vehicle.
Well, and finally the subwoofer, the performance and the parameters and, of course, the price and dimensions not too big, we chose the JBL CS1204B . Everything is done so that the car can be returned to a serial state, no damage to the cabling and below. It is a new car with a 5 year warranty. The assembly took us 2 days and cost 1485eur / in price also car foils /.
That's a lot of money!