Bass is over-rated. Baka maging katulad ka ng mga jeep na may subwoofer ah
Monoblock pagalam ko is just 1 channel rather than the usual 4channel ampli. In other words, dedicated amplifier just for the woofer. Seps/Coaxials is powered by the HU. Yan pagalam ko, tama ba? Pero mahal mga monoblock to begin with eh
I drive my sister's M3 and it sounds fine naman. Stock sounds really has progressed a lot since the 1990s. Even without a woofer, you can tweak the bass from the HU (of course, the intended practice is to just use an equalizer to compensate for the car's acoustic deficiency but to heck with it..). IMO, I'll rather get the additional cargo space and remove the woofer. If not, I'll go save the money for a better sep.
A budget for 3-4k is really stretching it. First hand purchases might be too expensive so go find some decent second-hand ones.