Hmm... interesting questions. First question for you would be this: have you HEARD the Kustoms? Now, granted, I've used some Kustom monitors that were nice because they were really light and small, but sound-wise, they were pretty pitiful. So, you first need to make sure you'll be happy with the sound.
Next question. If I remember correctly (I've used some of those EV subs... pretty nice), the Force subs have a built-in passive crossover. Do the Kustoms? If they do, you can do like you said... if not, you may need to consider some alternatives. The Kustom cabinet will most likely distort if you run low frequencies into a 12" woofer without a crossover. What are you using this for? Venue size/number of people? Type of music?
As far as volume differences, this may actually work out decently. Because you're using two 16 ohm mains with a 8 ohm sub, the mains together get about the same power as the sub. This volume difference is more of a concern if you were talking about monitors or 3 mains.
So... answer the above questions, tell me about your amp, and I'll tell you what I think. Are you mainly trying to go for a wider sound distribution from your mains?
PM me if you don't hear back soon.
