I typically stick to 6 1/2 inch Pyle Blues (http://www.parts-express.com/pe/showdetl.cfm?partnumber=267-082) You cant beat the sound quality for the price. Im on my 3rd machine set with great results including one set on an extra amplifier. You will have to enlarge the right speaker hole due to it's original smaller size for the installed factory tweeter/mid speaker. I do think the matching speaker sets improve the balanced sound as well. Many other Pinsiders have used these same speakers.

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You can wire them in series and reach the 8 ohm rating with 2 speakers and this is what I suggest due to the board being a mono set-up. Parallel wiring the 2 speakers will give the mono amp board a 2 ohm load and most likely cause pre-mature failure over time. If you aren't familiar with these wiring set-ups here is a good diagram. It's pretty simple. http://www.triodeamplification.com/Speakers.html