The A500 has a different filter circuit to the more modern Amigas, so bear this in mind when using the filter. And true, you can't compare it to cd or sound card quality as there is no oversampling with Paula for starters, you just get the pure output. So in reality you can't really compare it to anything.
With all this test gear you're going to throw at it, and with your downsampling, I suppose there's no chance of a listening test then? :-P