[X] Aspect: Visions
[X] The Brass Cube
[X] The Revolver
I think the opportunity cost of picking the Brass Cube is minimal. The Lantern seems vaguely sus, and the Red Athame is a melee weapon. It might have hidden potential, but we're already getting the necronomicon and it'd probably be unpleasantly gothic potential; and in terms of immediate safety, I don't think the Athame particularly helps if the revolver is already taken.
picking a Potential-Focused Unmystery Box over a Weapon We Probably Won't Use or a Suspicious Lamp does not mean you're committing to focusing on it! If anything, I think the right plan with Brass Cube is to totally ignore it for a long time. The greater his other powers, the greater the precautions Solomon will have the latitude to take in the process; the lesser the endangerment to innocent lives, and so on. Yes, if we take the thing we want to benefit from it eventually, but that doesn't mean it has to be before graduation. In similar vein, hopefully the growth of Solomon's aspects and such will make it cheaper to unseal the cube.
I vote for visions because it's the least Horror King-y.
[EDIT: hm, it appears someone else voted for the same things first.]
[X] Plan May you have Eyes..