High power amp for monitors?

rwwear is right on.

For your benefit:
Assuming 91db speakers
1 watt = 91db
2 watts = 94db
4 watts = 97db
8 watts = 100db
16 watts = 103db
32 watts = 106db
64 watts = 109db
128 watts = 112db
256 watts = 115db

With that said, it takes double the wattage to gain 3db. It take 10db to get a perceived doubling of power. So, with exact same "quality" amps you won't hear a difference.

Your 150 watt rating is continuous. That is a huge amount of power to put out from any amp continuously. Try a different amp for fun. You may be surprised to find some 30 to 40 watt amps that can hold their own against a 200 watt amp...(another argument)

IMHO, I'd upgrade the speakers before getting a 200 WPC amp. I think you will get more of an improvement that way.

If I'm correct I think you would have to go to a least 250 watts to get a 3db increase in volume. I suppose this is why you want a more powerful amp? If it is SPL you're looking for, you may want to rethink spending the money on an amp and consider more sensitive monitor speakers?

Well, they have average sensivity(91db) and are 8 ohm, in a small to medium size room(12X18), but I have a chance at getting a Rotel 991, and a RB 1070, for almost the same price, but the power recommendations for these speakers are up to 150WPC, so I am not sure if I should stay within that guideline, or opt for the 991.

There is no such thing as too much power.

You have to double power to get a 3db increase in output. Shouldn't make much of a difference on a pure power basis.
Of course, the amp may have other properties that you like better.