I'm kicking around getting something else to compliment my current set of monitors which are the Yamaha HS-80Ms and would like recommendations. I do have a treated room that is approx 15x15 (yeah, I know) and whilst I've come to learn the Yamaha's pretty well I can't help but wonder if it would be a good thing to have a secondary monitor set to reference my mixes rather than my current option of using my headphones. I mostly deal with rock music, largely guitar based (electric and acoustic) and my budget is probably $500 or less. If I left out any pertinent details, please let me know. Thanks.