No offense but the PSB Subseries 150 at $499 seems like a bit of a toy compared to similarly priced alternatives. It only outputs bass down to 26 Hz and you want a sub that goes down to at least 20 Hz.
I suggest you return it and purchase either one of these SVS subs, the PB-1000 that has a port, extends to 19Hz , has a 10" woofer and is 15"wide x 19" high x 20" deep or the SB-1000 that is sealed, extends to 24 Hz, has a 12" woofer and is 13" wide x13,5" high x 15.5" deep. Both are $499 and have both line level/RCA inputs and speaker level inputs.
Here's a link to both subs:
Either of these subs will provide significantly better bass performance than the PSB 150 and the connections to your PrimaLuna integrated via the speaker terminals are legitimately good methods.
I'm assuming you don't want a large sub and both of these would be classified as small to medium sized subs, which should be able to be located in an inconspicuous location or even as an end table in your room.
They both have the typical adjustable controls for volume, crossover frequency and phase to allow for a smooth integration of the additional bass power, detail and dynamic impact with your PSB speakers.
You haven't mentioned if you use your system for just music or for both music and home theater. If it's just for music, the SB-1000 may be the better choice since it's a sealed design and blend well in its detailed character with your sealed PSB Synchrony 1Bs. If it's for music and ht, the PB-1000 may be a better choice since the bass extends a bit deeper on both music and ht.
If you have the room, utilizing two subs usually provides even further significant bass improvements in the areas of bass smoothness, detail levels, power, dynamics and impact. SVS is currently having a small sale of $50 off when two identical subs are purchased, so a pair of either would be $950.