Which component upgrade will produce more BASS?

Agree with Bill. Change the speakers. By lower mid bass I take it you mean kick drum (rather than a subwoofer) is your main need. If you want to go serious then get something with at least a 10 or 12" woofer (I would add to check the voice coil size and that 3 inch is a minimum) - someone mentioned JBL for example. FWIW-bass is generally best in a sealed box until you get into very large speakers.

Large pro speakers use reflex ports to get higher efficiency by putting a bigger drive motor in the same size box whilst most smallish speakers use the port to get bass extension. Usually ported bass extension results in some loss in bass clarity or a tendency towards "one-note" bass (it is a trade off that is well worthwhile for the better WAF factor of small speakers). There are some excellent transmission line designs by PMC. Dynaudios are also pretty good - even their smaller woofers in their Appolito designs can move some serious air. Again I assume (and I could be wrong) that you want that room loading punchy feeling you get from a kick drum rather than just a boomy powerful lower mid bass. Perhaps you could give more color to your needs - for example what have you heard that made you feel you were missing lower mid bass?

Rauliruegas,

Good link. I did not read the entire thread but I realize that I do share your views about the terrible amounts of distortion from typical bass in both subwoofers and most speakers. This is why I recommend large woofers with large motors with large diameter voice coils working with a short coil in a large magnetic gap (the most linear type of woofer). I did not mention it above but the vast majority of woofers are the lower cost small diameter long voice coil in a short gap and which means they are pretty much non linear over their entire operating range and resulting in highish amounts of IMD distortion and harmonic distortion.

However, I think this goes far beyond the simple request of Ziggy and don't want to divert us from that goal but nevertheless I agree with you wholeheartedly.