I own both the PB-1000 and Monolith THX Select 10, as well as previously owning a pair of PB12-NSD's and PC12-NSD's. With the SVS PB12-NSD currently being $500, I would go with the PB12 over the Monolith 10. Now the Monolith is a bit smaller, but it will choke a bit when LFE stresses it and it will chuff. However, if you can't fit a PB12 due to space constraints, then I would opt for the Monolith 10 over the PB-1000.
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. Sony CS3/CS8/CS5. QA 3020i/3090Ci, 2020i/2000c. Monolith Cinema 5bs. Polk RTiA1/CSiA4. Other Audio>Sony MDR-Z7m2,V6, XB900n,700,XB32,XB31. Sennheiser HD58X,HD4.50,PC37X. HiFiMAN HE4XX. Philips SHP9500. SIVGA SV004. SA HDTwo. HyperX CloudPS4. TB Elite AtlasAero. LG FH6,RK8,RK7,PK5. HK Onyx Studio4(x2).