Matt posts on the AVS DIY boards as mtg90.Actually, I saw a person named "Matt Grant" on another forum
It seems he makes the pcb board for the 1299. But I dont know how to contact him yet. I havent tried that forum though...
Here's an active thread of his: https://www.avsforum.com/forum/155-diy-speakers-subs/2226642-v-b-s-s-diy-subwoofer-design-thread.html.