Well for me that’s easy. One SVS over any amount of polks. I have 2 pc pluses AND a 505 as a midbass module(traded my friend for an sb2000 who needed a sub more than I did.
No matter how many 505’s are in the room, they will never extend deep enough for movies, and when pushed they can make some bad sounds. The SVS is better in every single way. Power, control, linearity, and in and on. The 505’s can be ok enough if you’re on a budget but no matter how many you have, IMO the single “better” sub will win.
To answer, get the SVS. It if you have 2k to spend, there are plenty of options.