I had band practice the other night and have come to a few potential realizations. The dually(s) is/are incapable of fulfilling my needs. Class D heads are incredibly quiet given their wattage ratings. I am in a string of bad luck with both pieces of gear and just flat out have defective equipment.
My lead player was using a 5150 on the high gain channel with pregain between 3-4 and master around 5. I could keep up with my rig but I was absolutely tapped out. Any more bumps to either gain, bass or volume and the power amp started to clip like crazy. On top of that, I had some low lows but nowhere near what I can get out of almost every other rig I have had.
I understand that Duallies are not subs but I think the small form factor is incredibly limiting with bass response. On paper that may be different, but in my hands on experience I have to say that with or without my HPF the bass response is about the same. I could be doing something wrong, but I can't afford to buy another dually if it is not going to get where I want. I am abandoning ship on that idea. I'll probably look for another used 810 and put everything to rest.
I tried all of the ideas that you guys offered and I do indeed have much more potential volume from the preamp, but I'll be blowing speakers like party poppers. After additional rig swapping I have also concluded that my 2000s/215s is substantially louder and deeper than 2000s/dually. It just is. On paper that shouldn't be, but IRL it is.
This isn't to say I am dissatisfied with my dually. I love the cab. It sounds great and I think for any other application I will be set up with more than what I need. It just isn't enough for loud, down-tuned metal. Julian was right. I should have just saved for a 1515/66.
I am not really sure what if anything is defective because at low to moderate volumes everything sounds great.
The prolite is only marginally louder than the Sunn beta when I would take that to practice. I know that the betas are stupid loud for their power rating. I just don't understand why I feel like the prolite is so quiet. Both of my tube amps are also far louder.
The speakers sound incredible. But I am giving them enough juice to potentially fry them and they still aren't getting me to where I need them to.
This is why I think these cabs and class D just aren't right for me.
I think I am going to sell my rack and a few cabs, pedals and maybe a few basses and buy an SVT or a Super Bassman if I can swing it without my GF castrating me.
I'm keeping the dually for my home rig and to take to much quieter jam sessions. I may still use it for practice with my metal band and will still use it live until I can wrangle another 810.
I'm feeling pretty frustrated by the situation.