Have you thought about just adding 1 Signature SUB 2? I haven't seen the MSRP on that bad boy yet.
But seriously...2 SUB 25's? Are you going to hook them both up to 240v circuits? Please have us over afterwards...I drool at the thought of adding 2 SUB25's into my set-up. Can't imagine having a room big enough that wouldn't be immersed in bass at the bat of an eyelid with those monsters integrated.
You sir, are a bass fiend. I like you a lot.