http://www.parts-express.com/resources/woofer-selection-guide.cfm
Sort the guide by woofer size. Measure and match the hole opening, approximate volume of the speaker box and the -3db cutoff specified by Sapphire to something in the guide. Look for 4-ohm models, of course. Also, if you know the sensitivity rating (87db or similar), shoot for the same rating.
In order of importance -
DB match - get it wrong and the speaker will sound dark or bright.
Impedance match - get it wrong and the crossover frequency will not be correct.
Volume / F3 match - Will affect bass response.
As Sapphire is long gone, aftermarket replacements are your best bet. You might be suprised at how good a shot in the dark will sound. PE has a good return policy if you take care not to mess up the drivers during tryouts.
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