Duke did a very nice job of summing things up. I'd only like to add that there are different ways to increase "power supply reserve" aka filter capacitance. Just as there are different sized "buckets" with varying "fill & spill" times, caps are the same way.
One can use a few high value capacitors or a multitude of lower value capacitors. In general terms, most larger caps have a higher series resistance. This increases both the amount of time that it takes to charge and discharge, making them less effective than a cap with a faster charge / discharge rate. That is why many manufacturers have gone towards using multiple smaller caps rather than a few big "cans". As a side note, smaller caps are also cheaper than big caps, so it helps them out in cutting production costs also.
Having said that, adding capacitance not only offers more reserve, it also offers more filtration. As you increase capacitance, you also have the benefit of reduced ripple and line noise.
Not only can one increase capacitance, but one can also alter where the "reserve" is located. While most amps have a large quantity of reserve located in one central location near the AC input / transformer, McCormack amps place a bank of smaller ( yet still good size ) caps very near the output devices. Since this is where most of the power is needed, having them closer to the point where the power draw is coming from may increase dynamic impact and reduce noise generated within the circuit itself.
Many "tweakers" do something similar with digital circuits. They take and place caps right at the voltage feed into various IC's within the circuit. This helps to clean up some of the garbage that is pumped both in and out of the IC and helps to stabilize the circuit just a hair.
If one were trying to obtain optimum performance, i would try increasing filter capacitance at the location it is being drawn and at the central point of supply near the transformer and do so using a variety of capacitance ratings. The more staggered the capacitance ratings amongst the caps, the greater the ability to filter a wider variety of noise on the line AND stagger the charge / discharge rate of the power supply. This results in a lower noise floor, reduced ripple and a more dynamic presentation.
Having said that, one must be careful when going "gonzo" on increasing capacitance. One can create problems in terms of the caps "burping" upon turn off and possibly strain the transformer upon initial power up. Like anything else, any modification or upgrade should be fully thought out before attempting or you'll learn the hard way that adding / changing parts DOES require knowledge of how the circuit works. Sean
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