If you need to test for distortion, then you'll need a high resolution (24 bit) ADC. A USB interface with some voltage attenuators (to reduce the amp output voltages) should be sufficient.Mainly looking to do functional and qualification testing and high level troubleshooting. As the amps are modular, will not be doing component level diagnosis or repair. Figure if I can run a decent power vs. distortion test, will cover a lot of territory. Currently just generating signals out of REW and Audigy sound card.
Resolutions of digital scopes goes to 16 bits max and probably is insufficient for distortion measurements. However, if you need high frequency bandwidth, you may need both.