Do I have to use my prequalification amount on the Audioengine website?

Yes, if you apply for financing on the Audioengine site, the prequalification amount is tied specifically to audioenginedev.wpengine.com. If you wish to pay with Affirm at other merchants, you can apply for credit on any other site that uses Affirm, or you can apply for a loan directly on the Affirm website to receive a virtual credit card.  See here for details.