I believe one thing you are not considering is the electrical upgrade needed to run the power you want. Big 3 is a given. Going 1500-2000 could mean an extra battery, just depends on how much draw the specific amp is and how powerful your alternator is. Going 2000-3000 usually means a ho alternator in addition to the other two. In other words 3k means $5-700 easy in electrical upgrades. Also bear in mind quite a few amps are strapable. Meaning you buy 1 sundown saz-1500d now for around $250 used. Later you buy a second, link them together and have 3k. Or of coarse they do not have to be strapable. You buy a hf or mb quart and run to the 2 subs. Buy another and run to the other 2 later. Just bear in mind whatever amp you go with. If you don't maintain the voltage it needs, it will end up frying so you just wasted the money. Personally I would buy used from a member with good feedback. You can often get an amp for 1/2 the price of new. Which would put much better amps within reach.