Gotta agree with bass on this one. You'd be much better served by adding the SI18's. Honestly, I'd add those, then I'd try and sell the XS15s and build more 18s.
My room is about 4300, and my single THT does ok, but I'll be building another in the future.
Good luck Alan!
HT: Denon 3808ci | NAD T763 L/C/R | Klipsch KLF 20s | Klipsch KLF C7 | Klipsch KLF 10s | Klipsch KG 1.2s | 24" THT | Bash 300S | Panamax 5300PM | Panasonic 55VT60 | Oppo 103 | PS4 | X1
Family: Pioneer SP-SB23W | Pioneer Elite Pro 111 | PS3
2 Channel: Yamaha R8 | Cambridge Audio D500SE | Klipsch Heresy HBR