Passive / Unpowered monitors may be a decent choice for you if you already have a power amp. Generally unpowered monitors are quite a bit cheaper than their powered counterparts. If you want to buy unpowered monitors you have to worry about a few things though. First, you need to make sure your amp will work with the monitors, and ideally not blow them up. You also introduce more cabling into your recording studio when you need a seperate power amp. You need to make sure the power amp gets proper cooling and that you are using the correct cables to connect things up.
The Yamaha HS80M - Yamaha's new powered version of the NS10M monitor:
Remember that amplifiers have a sonic character of their own. Just as you might combine the sonic characteristics of a microphone and preamp, you need to consider the combination of the sonic character of your reference monitor and separate amplifier. In other words, the same passive / unpowered monitor will not necessarily sound the same when powered by different amplifiers.
A general rule of thumb when searching for amplifiers to drive your passive / unpowered monitor is to purchase an amp that delivers twice (2 x) the wattage necessary for the monitor (this allowance is for headroom). So if you need 300 watts at 8 ohms, purchase an amp that is rated for 600 watts at 8 ohms. Again, this is just a rule of thumb and is not necessarily true in all cases.
