Sony Xplops are sh!te,I guess they're 6x9's? You'd be better off with a decent sub and amp in the rear,I use Infinity Reference Speakers in my Mk3 but use a crossover with them to take out all the Bass and the sub in the boot balances out the sound nicely. Are you running an amp at all or relying on Headunit power only? If you are just using a headunit try running an amp through the 6x9's first,then if it still sound bad you have the amp to run a sub from.