Howlin' Dog,
You have a halfway decent amp already, so I would definitely recommend buying passive monitors.
The amps built into most of the cheaper powered monitors are really not that great anyway. Active crossovers and bi-amplification is
potentially much better, but if the electronics or most especially the speakers drivers are cheap, a comparably priced passive speaker with higher quality components will have better performance.
Think of it this way. Your amp is rated THD=0.05% from 20Hz-20kHz. The average 8" midbass driver like those used in the common passive and active monitors (Mackie, Event, Yorkville) have numbers like THD=1% 100Hz-1kHz and THD=5% 45Hz-100Hz at only 5 watts input power!
Amplifier specs are usually quite misleading and don't give an accurate representation of how they behave actually driving a complex speaker load. So lets just assume the real world specs of your amp are a whapping 10 times greater than what Yamaha claims - THD=0.5%. This is still 10 times smaller than the speakers low frequency spec for very moderate power input.
From microphone all the way down the audio chain, speakers are by far the weakest link. They are your ears to your music, so try to get the best you possibly can. Even if you pay $10,000 for monitors, they will still likely have the worst specs - unless you have absolute shit for equipment. Whatever you were considering to spend on powered monitors, spend the same or more on passive monitors and use your Yamaha amp.
What would I suggest?
How about the Quested VH2108 at $1900 a pair? (Quested makes top of the line pro monitors)
Can't afford this? How about something far better for $1700?
Here's a kit from Speaker City. Choose the 9900 "Revelator" tweeter option.
http://www.speakercity.com/scan_mtm_18.shtml
The ScanSpeak drivers used in this kit are some of the most advanced today. If you built this kit you would have a world class set of monitors.
Still too much? How about something still better than the Quested for $1300?
The Raven
7 kit.
http://www.speakercity.com/sc_kits.shtml
Still too much? How about something comparable to the Quested for $800?
The ScanSpeak 7 kit.
http://www.speakercity.com/sc_kits.shtml
What's the catch? None really. These are outstanding kits with exceptional drivers and crossovers. All of the ones I have mentioned are far more linear and accurate than any Mackie, Event, Tannoy, Yorkville or other such monitor you can buy. My only criticism is the quality of the cabinets. But, for less than $50 in materials I can show you some relatively easy modifications to the cabinets to make them world class as well.
barefoot