High power amp for monitors?

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?