There is nothing wrong with the S2 infact the idea was to add a second but it's more than I want to spend.What is wrong with the S2? If you are talking about this model; REL S2 Subwoofer | REL | Audio T
That model should perform better vs. XLS200 and for that price i assume the build quality is better too. Larger cabinet with dual 10" drivers and design is similar to older XLS300PR which was a step up from XLS200, especially with movies.
Do you have space problems as you are looking so tiny subwoofer(s)? You have wanted add for Arendal S1 and such an awesome system other wise, i am little puzzled why would you settle for cheap small subs or are you looking these for other room?
XLS200 performs best in very small room. If in doubt always go larger one and add second one in future.
Have you thought about dual BK P12-300SB? Price jump from XLS200 is small and the larger cabinet & driver will put a smile on your face with movies. You might get lucky with grade b* model and save some bucks, worth to ask from BK. Performance should be very close to XXLS400.
* BK Electronics P12-300SB-FF Satin White with White Grille | eBay
My choice also comes down to aesthetics along with obviously performance. I have Just purchased smaller fronts which I have spread further apart I have the S2 next to the left front but the right side looks empty now so wanted to fill the gap with a second sub but wanted small to compliment the newer smaller fronts for that sleak look.
I looked at the options you have mentioned already both the XXL400 and P12-300 are over budget.
I have been in contact with BK and was told that dual XL200 Will perform equally to single XXL400 With added advantages.
They have dual b graded XL200 available so was considering them.