A rough estimate would be comparing what the PB13U can do in sealed mode versus the XV15 over at data-bass.com; judging by PSA's estimates of the XS15's output capabilities, I'd guess it's comparable to the SB13.
One other estimate of the SB13's abilities I've seen mentioned by SVS is that it has a 6dB average advantage over the SB12-NSD, whose measurements are also at data-bass.com.