here is my setup, cause i also wanted something light, but well heard. i was even able to just use the trunk shelf for my two amps and 12" kicker sub. the sub is shallow mount 12" kicker tcvt sub with an enclosure. i bought it for about $150 on ebay. it fits perfectly snug and doesnt even touch my 6x9s, i slit a piece of rubber tubing and wedged it to the top of both sides of the sub box, doesnt move at all.
also, if you want even lighter and dont want a sub , just get a good 4 channel amp with some good 6x9s to give you some kind of bass. my polk audio's gave some very nice bass by themselves. my car sounds awsome with 3.5 jl audios, and 5.25 jl audio coaxials and polk audio 6x9s. and i still retain the stock look without any kind of components.