[pierreye]

If OP only want to improve mid bass performance and lower distortion at higher level, better replacement underseat sub will fulfill this requirement. Only way to get lower sub frequency (below 50 hz) is to get trunk sub. No other way around this.

As OP had HK system which personally what I have in my F10, the only upgrade path he had is changing the speakers and underseat sub. Done that and I'm quite satisfy with the improvement. The OEM HK speakers + sub sound muddy when playing at louder level (more than 60% level). Lower volume I don't hear much different.

Note to OP: HK underseat sub is 8 ohm. Don't get 4 ohm underseat sub. Only underseat sub that match the spec is either Bavsound Ghost or Alpha One which is 7 ohm. My understanding is they used 7 ohm so that the driver will sound slightly louder compare to stock. So far OEM HK amp have no problem driving 7 ohm underseat sub.