It looks as if your question got lost a bit in the other discussions. I would not personally characterize this as a lateral move at all. But, I think it depends somewhat on what you are looking for. PSA ported subwoofers have port tunes in about the low to mid 20Hz range, depending on the model. But, they are typically stronger than comparable SVS subs in the mid-bass range (from about 60 to 100Hz). Tom doesn't use very low port tunes, which is a primary factor in determining low-frequency extension, and he deliberately concentrates more amplifier power in the mid-bass range than SVS does.
The lowest frequencies, on the other hand, require several factors to achieve. Those factors typically include: larger cabinet volumes, lower port tunes, more amplifier power concentrated in the low frequencies, and stiffer drivers (woofer cones) with greater motor strength. So, the two subwoofers will offer significantly different advantages. In my personal opinion, most people with good quality dual subs will have plenty of mid-bass SPL. Once they have that, they will probably upgrade for more low-bass SPL and TR (tactile response). And, the PB4000's will have a very sizable advantage in that respect.
You can find CEA-2010 test data for the PB4000, but you can't find comparable test data for most PSA subs. Max output is never listed, by individual frequency, for direct comparison purposes. What is shown, though, is the native frequency response of the PSA subwoofers. And, if you compare those, to the native frequency response of other subwoofers, you can see the difference in low-frequency extension.
For instance, you could look at the V15DF, on the PSA website, which is probably the closest current example to your sub. If you click on the Performance tab, you will note in both the frequency response graph, and the compression graph, that the sub begins to roll-off quite perceptibly at about 30Hz. By 20Hz, it is down by 3db, and by about 17Hz, it is down by about another 5db. So, the sub loses about 8db between ~ 30Hz and ~ 17Hz.
If you compare that to the PB4000, on the SVS website (clicking on Tech Info), you will see that the sub is almost ruler flat in the Extended Mode (16Hz port tune) from 30Hz out to about that same 17Hz. That gives the PB4000 about an 8db advantage at 17Hz. That is an enormous
difference, which should be perceived as additional bass weight with most content, and which should increase both audible and tactile low-bass special effects in movies. The subwoofer also has some user-adjustability features which can help to tailor the sound and FR somewhat to your room and personal tastes.
The big SVS ported subs are low-frequency specialists in Extended mode. But, with duals, they should also have enough mid-bass SPL to satisfy most listeners. That is why I say that, depending on what you are looking for, they are not a lateral move, in my opinion. For me, at least, they would constitute a pretty serious upgrade. I hope that you find this analysis helpful!