Which component upgrade will produce more BASS?

If your favourite music is rock from the 70s-80s, I would go for a JBL. A used JBL 250 TI for example. That would produce the type of sound you may like and fit for this type of music. But I would, anyway, buy a speaker with 10" woofer.

Use of subwoofer is a good idea. One of the way to build a good sounding system is the employment of subwoofer. But still there is a problem: integration into system, which already has coloration which the owner do not like. That is a difficult task for most people. In general, I think, that most home systems are just sounding crap. Especially, if one takes into the cost of the system. Most people following the fashion of particularisation through building up the system from specialised and dedicated pieces. It is just extremely difficult to build up a naturally sounding system. Most so called high-end systems have a very owner specific coloration, with which the owner itself is not completely happy. Hence the constant changes. My personal experience is that the best way to avoid the possible merry-go-round is to go for as simple system as possible. In this term, given the extreme importance of loudspeaker-amplifier link, the best way is an active loudspeaker. It is just amazing how easier to get right the sound of with an active loudspeaker, and how much once that is right you can get rid off spending on such complementary colouring things like cables, etc.