As an aside, to help with cost and luggability it will often sound better having just one well-placed sub as opposed to two spaced either side of the stage, which can (and does!) cause phase issues which can actually DECREASE the perceived bass levels.
Downsides of course are possible loss of visual symmetry and having to take stands for your tops, as well as finding somewhere to position the sub (centre-stage is perfect, but obviously not practical most of the time), but if you can get it right it's one less big, heavy box to cart in and out and one less amplifier channel needed.
For a decent sound with a bit of headroom I'd be looking at 800-1000W for the tops and not much less for the sub(s), properly crossed-over; if I were building from scratch on a budget (and actually will be soon!) I'd want something like a 2x800W power amp, crossed-over at about 70-80Hz (experiment with this, it's worth it!) with one side feeding the homebrew EV Delta 12" speakers in Celestion cabs I already have (300W RMS handling, eight ohms) and the other side feeding a single 600W-ish four-ohm sub.
Of course if you're buying active you're slightly more limited, but brand-matching should ensure that each speaker gets fed with exactly the power and frequency range it needs. I'm actually in a minority in having got (what I thought were) quite pleasing results from a Mackie SRM setup, but these were the original RCF-made Italian ones; from what I understand they've gone downhill since production went Far-Eastern.