Did you try to contact Amphion?
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On Peerless Drivers the number you need is the catalog number in your case 850628, however doing a search it does not come up as a good number, are you sure you have it correct ? It can also be that its a custom made driver by Peerless for Amphion in which case your only source will be Amphion. If you can't get a response from them, you will have to settle for a stock number that comes close.
A search for the Amphion speaker reveals that it one of the older multi-layed poly cones under the CSX line, a possible suitable replacement driver can be found Perless 8 Inch Driver
Best of Luck