if youre willing to add an amp & 'small sub', than might as well get a 'normal' 8/10/12" sub. between the 2, there really isnt enough difference in weight to be concerned about in a daily driver. we're talkin about a few kilos here..

reasonable sound is subjective.. sure you might not be an audiophile but youve said yourself that youre not happy with the quality of sound, lack of subbass & how loud it can go.. why? cus youve basically hit the limit of the gear (just a HU & speakers, no amps or subs) & installation (sd'ing, outside noise reduction/proofing to mask your exhaust, speaker positioning, tuning, etc). you'd have to address those if you want improvement.

youre simply expecting too much from too little.

guess you gotta ask yourself whats more important: a few kilos of weight saved or a sound system you like the sound of?