your thinking is flawed...

loudness does NOT increase the power the stereo can produce, indeed if anything it decreases it due to distortion in the loudness curcuits. The '%' of change to the frequency curve of the sound is largely dependant on how 'crude' the loudness curcuit is...

Most trble/bass controll will give at least 12bd of boost/cut - now thats 4 TIMES the volume....i have seen loudness curcuits that produce similar boost - but at a price...

basically what your in danger of doing is raising the output level of the pre-amps to the point of creating distrortion in the power amps...and that damages speakers....

...now back to how loudness works ALL it does is to boost the treble and bass...it DOES NOT increase the ability of the stereo to produce volume..indeed loudness was pioneered for listening at LOW volume (a condition where speakers tend to loose treble and bass ability ..particualary in single cone configurations)...

so back to the question if you want MORE volume and QUALITY then your going to have to upgrade your amp and speakers...if you want to change the tonal balance of your sound...use the tone controls...

If you want more bass content to the sound adding an amplifier will have ZERO effect..it may raise the volume but it will not, proportionally, produce more bass (or treble). The add on here is that of course if you control the amp through (crudely a bass pass curcuit) or better an active crossover and then match it to appropriate bass speakers ( i wont use the word sub-bass here as most speakers sold as such for cars are not really sub bass - and you wouldnt want to be in the vehicle if they were....) then you will produce more bass...

If its done via an active cross over and a second amp set driving appropriate high frequenct drivers (try compression horns for that real ZING) then you can boost the high frequency content too....but at this point you wil be wanting to add mid range amps and speakers as well...and you have just spent a lot of money!....

so if you want this sound cheaply...replace your head unit with an aftermarket one with XBAS or the other marketing ploys they are now calling the loudness curcuit (and they sound HORRIBLE if they are turned up!)