Reality is you're better off trying to either A. Find a used Castle for sale, or B. Selling your castle and going with a pair of different subs.
It is pretty much never a good idea to mix two different subs together, even if they're from the same company.
The Empire is a great sub, but its going to be a lot different than your castle. The castle was a great sub for digging deeper, more of a HT sub than a music sub. Not that it was bad at music, it just was better suited towards HT. The Empire has gained a reputation for being VERY strong in the midbass region, so its going to feel like a wholly different sub than what you're used to. Some scenes in the lower frequencies you'll expect to be like the old sub won't be performed as well, but then others which are more midbass will probably be done a lot better.
If you're interested in other brands, you could pick up a pair of eD A5-350's or A7s-450's and you'd be sittin very pretty.
Klipsch RF-62 L/R, RC-62 C, Energy C-100 SL/SR, HSU VTF-15H, Marantz SR5003.
"All speakers sound good when you're on a coke binge in the middle of an orgy." - Heebdawg16