Just buy another sub-1200 and put it right next to the one you already have. That will give you a 6dB increase in output. Your room/garage is a really large space to fill, so unless you increased your budget dramatically, you aren't going to be bumping that size space.
Subs>RBH I-12e, I-12. HSU VTF-15H mk2. Monolith THX 12(x2)/THX 10(x2). XTZ 1X12. SVS PB-1000(x2). Speakers>JBL Studio 270/235c/230/225c, Arena 130/125c. Sony CS3/CS8/CS5. QA 3020i/3090Ci, 2020i/2000c. Monolith Cinema 5bs. Polk RTiA1/CSiA4. Other Audio>Sony MDR-Z7m2, MDR-V6, WH-XB700, XB32, XB31, XB22. Sennheiser HD58X, HD4.50, PC37X. SIVGA SV004. HyperX Cloud PS4. TB Stealth300. LG FH6, RK8, RK7, PK5, PH4. HK Onyx Studio4(x2).