How did you manage to get the Betas for $449 a pair???
That's 150 less than what it shows online.
Did you manage to do a price match, or is an open box deal?
Regarding your question:
I think it is a good idea as long as you replace the center with something that matches the Betas.
It is going to be a brilliant idea if you also replace the surrounds and the subwoofer with something at the same level.
Another advice is that if you are planning to use the cubes as center between Betas, it will be way much better if you simply disable center channel in your amp. That way you can keep the good looking Bose cubes on the tv, but use the very good Betas for center channel information.
Sebi