No disrespect. I've been playing guitar for 23 years and use to play out with bands years ago, and know my guitar gear. I'm definitely new to bass gear. All I can say is I don't like these cabs compared to other stuff I've played through. My brother has an 810 SVT cab that the PF500 sounds fantastic through. It also sounded killer the ought the 210av cabs I use to have. I'm just not into the 15's I guess and yes, they flub out when pushed, regardless of EQ settings. Sure I'm new, but looks like I'm not the only one on here with similar experiences with this cab.
No disrespect, but I think you are doing something wrong, or as has been mentioned the seal on your cabs needs to be looked at.
I say this because if you are comparing the 115's to the 210av's and the 210's have a better output, something fishy is going on. The 210's are medium duty at best.
What are your exact EQ settings? all the knobs, and the ultra switches please.