Is the new sub going to be in addition to, or in lieu of, your Klipsch SW-450? If it's the former you should try and get another bass reflex unit in order to make integrating the two easier. If you're replacing the SW-450 then all your options are still on the table.
How big is your room? That's a pretty critical piece of information when determining what subwoofer(s) might work best.
Originally Posted by rmc29615
Ive often read that you want 200 watts rms minimum for a decebt sub
Not necessarily - there are many factors that come into play...
To an extent subwoofers live by the credo "bigger is better", and that includes the amp. However, the design, driver efficiency and amp class play key roles as well. As a general rule, acoustic suspension (sealed) require more powerful amps because the air inside the cabinet tends to impeded the drivers movement. One of the best ways to overcome that is raw power. Bass reflex (ported) don't have that same resistance to overcome, so they can often get by with amps that have less power.
A drivers efficiency rating also comes into play. An inefficient driver -- for arguments sake, one with an 85dB rating -- requires far more power to drive then one with a higher efficiency, say around 90dB. Amp class is yet another consideration; class D amps put out closer to their rated power then class A/B amps do, so if a class D amp is rated for 200 watts and a class A/B amp is rated for 250 watts the driver itself may end up getting the exact same amount of power due to the efficiency differences.
That's just an overview of course, but it does lend some insight into other areas that will effect how much power an amp needs to provide in order for it to work properly.