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.