The cheapest option is to buy a cheaper flat, active subwoofer and dump it under one of the front seats (whichever one you prefer). Hook it up and it will provide some of that low-end sound you've been missing. This Pioneer
is a good option and receives some praise. More powerful but also larger options include ones like this Infinity
, but it also comes with the added pro of having speaker-level inputs (no hi-lo converters and you can keep your head unit).
If you want quality, you're going to need to look at something else entirely, such as replacing the speakers all round, getting a better designed subwoofer in a proper enclosure and installing amplifiers for the speakers and the subwoofer. Easier options exist (such as the Focal Solution 25A1
) where you can just install good speakers and plug it into the same subwoofer box for pretty excellent results.
I wouldn't worry too much about installing the best speakers in the rear doors or amplifying them. They're not the ones you primarily listen to anyway.
Baseline: Post a budget. It'll go a LONG way, since you get upgrades costing a few hundred bucks up to tens of thousands of rands.