High power amp for monitors?

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.

There is no such thing as too much power.

Examine the JM LAB Cobalt sensitivity and impedance curve. Also consider how loudly you listen and the size of your room. All these considerations factor in to the answer to your question.

If the speakers are low sensitivity (below, say, 88db) and if they have low impedance dips (say down to 3 ohms), and if you listen at volumes in the range of upper 80s-lower 90s dB, then you may reap benefits with a higher power amp that doubles its power output into 4 ohms. Otherwise, stick with what you have.

For further research, I recommend Robert Harley's "The Complete Guide to High End Audio". Every audiophile should own this book, IMO.

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.

The Cobalt specs I read online indicate a minimum impedance of just over 3 ohms. That's a significant dip, which indicates they require amplification that delivers lots of current into those frequencies at which the impedance dip occurs, otherwise those frequencies will sound recessed compared to the rest of the frequency band. Look for amplification that doubles it's power delivery into 4 ohms for best results. Buy and read the book I referenced above for more info about how to determine the power you require for your loudspeakers, your room and your listening preferences.

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?

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

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)