According to the engineers on here, that's right. I think it's the reason I never cared for mixed speaker rigs, too.Another 115! I usually don't like to mix speaker sizes. I've used lots of combinations eg; 210/115, 210/118, 112/115 and although they sounded really good, I just prefer the tone of either 112/112 or 115/115. Possible frequency incompatability? I don't know, just guessing, I'm not an engineer.
I love 15"s in smaller boxes because they push more mids that you can hear. Most of the bigger boxes get too boomy and end up with a hole in the upper mids, plus beaming (mid freqs in bigger speakers become way more directional and can't be heard as well off axis) plays a role in not hearing what mids you do have. So if you add another 15" with quiet mids to your mid-impaired current cab, you'll make it worse or no better, but if you add a different cab, you may get phasing issues.
What would I do? Hate to say it but I'd start over and build a rig around 10"s or 12"s.
