Need Advice on ABing Speakers

[Assimilator87]

I'm about to get some vintage Onkyo tower speakers which may or may not be better than what I have. What little I've heard about American Onkyo speakers is not good at all so I'd like to AB them against my R50s before selling one of them off. Since I'm a real amateur in the audio scene, I don't think I'd be able to tell which one's actually better. I may find the sounds different, but not be able to judge on quality. Do you guys have any advice on what I should be looking for? One thing I can definitely critique about the R50s is that there's a big frequency spike around 55-65hz and that really bugs me. Not sure what to look for aside from linearity.

Thanks everyone