Russian tested the SWR-12D4. There were two important findings. Claimed T/S parameters are very close to measured. Second, THD is very low, with subwoofer reaching 5% THD at much lower frequencies than other subwoofers from the same test (given some fixed voltage I assume).

They got 30.9Hz resonance frequency for the D4 model. The test of 10 inch model also will be online soon.

12 . Alpine SWR-12D4 -