Guess 4 x Rythmik FV18 setup is getting some votes.
Hmmmm..............
Will it be as ground shaking as 2 x JTR Captivator 2400 setup.
This is hard to answer without any context. But I would feel confident in saying yes. One BIG advantage will be in having two more subs. As shady mentioned before, FR is king when it comes to quality bass, especially when you get to this class of subwoofer. Having FOUR will go a lot farther in smoothing out the room than two. But I don’t subscribe to the idea that you can use a whole bunch of crappy subs and have it be better than a couple great ones. Again, in this class of subwoofer, they’re all winners.
It sounds like your friend might not have a lot of experience in this arena. I can say, if that’s true? He will blown away with any of these combinations. And if he’s really that critical about music, I believe he would already have some opinions on subs/speakers. So again, I believe he’ll be blown away no matter what.
I saw you got some nods to PSA. This is worth looking at too. Taking the whole measurements thing out of it, I know they’re are plenty of happy owners who, subjectively said they’re so much happier than when they had “XYZ” subs. Some also have in room measurements to support what they say so…take that for what you will. For Rythmik and the servo? Yeah it may not be relevant these days but they are excellent subwoofers so who cares…