I have the Harman Kardon audio option on my car and am very pleased with it...
However, as this PDF (above link) shows, the speakers under the seats are Woofers - not Sub-Woofers!
Particularly when playing uncompressed/lossless digital music tracks, I find the bass to be well-defined & clear - but many other people on this forum find the bass to be lacking... presumably because there is no Sub-Woofer!
I believe the standard audio system has speakers next to each seat, front & rear, but NO speakers under the front seats. These arrive with the various audio upgrades...
The H/K option adds two speakers in the top centre of the dash, the very stylish & clean-sounding Tweeters at the base of the A-pillars, the under-seat Woofers & the 400-watt DSP Amp.
In my opinion, it's an essential upgrade if you enjoy listening to recorded music (rather than the radio).