More is better, but up to a point. More can be better because of the filtering effect of the power cable. Generally lengths shorter than 5 feet sound worse than a 5 foot cable. But lengths greater than 10 feet sound worse than a 10 foot cable. Somewhere in between is usually optimum, but it all varies, both by power cord, and by the current required by the component. At the cost of a very good power cord, I would not usually recommend anything other than 5 feet due to the high cost and diminishing returns. But I would definitely recommend using a long power cord with monoblocks (or any amp), if by doing so you can reduce the length of speaker cables.
