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Cutting circles and making circular recesses (to flush mount the drivers) is a common task for the DIY speaker builder. I have never been able to get a router to make a great circle by hand. I can get really close, but I always slip once. There are bits for drill presses that can do this, and there is also the RotoZip tool plus circle cutter attatchment. Cost at Home Depot for the pair is around $75.
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The RotoZip can make circles from 4" to 12" in diameter and 14". (It can actually go a little smaller, but it is tricky.) It can also be used to cut little notches for tweeter connecting terminals as illustrated above. Getting better than 1/16 of an inch accuracy is a challenge. Consistency of use is key. I try and keep the plastic height adjustment knob constantly pointed in the direction of motion as shown in the left photo. The first pass creates a template for routing the driver recess. The bit depth is set to 1/16" more than the desired recess depth (as measured from the bit's tip). The cutting diameter is a little more difficult to adjust. I use a drill to make a pilot hole for the bit at the desired cutting radius. Then the RotoZip can be setup for the desired diameter and tightened up.
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An invaluable tool for making these adjustments is a caliper. Above is a dial type which measures 1/100 of an inch. This plastic one was about $15 at Home Depot. Just get one. Read the manual, it will tell you why a metal rod sticks out the end.
The second pass is to make the driver cut-out. It is just like the first pass except the bit depth is adjusted to penentrate the material. The pilot holes are actually necessary as I found the bits do not want to drill into MDF.
After all the circles are cut, a router makes quick work to create a recess.
A tip. You can go through bits really fast if they are not "properly" unstalled. The entire cutting portion of the bit must be outside of the chuck with a little bit of margin. Otherwise the bit bends at the cutting edge during use and breaks quickly. I learned this by going through 3 bits to make one 5" diameter cut in 3/4" MDF. The bits are over $1 each. The height adjustment on the guard should be used to get the proper cutting depth. After figuring that out, I was able to get almost 2 holes with one bit. Your milage will vary.
Another tip. A RotoZip will overheat if used outside when it is hot (like during August in Texas). It will take you hours to get the bit out after this happens. If you are that patient!