18" dual fi Ib3 build questions

I had a view questions about if I have enough airspace for these particular speakers. My room is about 3000 ft.³. There is a small storage unit connected to it that is approximately 500 ft3. Is this a big enough storage area forspeakership these speakers?

Also currently have a crown 1500 amp. Do I really need that additional headroom of getting the 2 ohm speakers Verses the 4 ohm? The power handling of the speakers is 520 wants at 20 Hz. My amp puts out right around there at 4 ohms. If someday I want to get two more of the speakers and move up to the 2500 amp then I'll be able to run 4 of them off one amp. What are your guys's thoughts on the additional headroom?

Cool. I'll probably go with the 4 ohm speakers so that the door is open for upgrading and adding two more down the road if I want to blow out the windows. The only thing that's going to be tricky is figuring out where to place the manifold at the front of the room since I'll also be building a new rack into the wall for my components. The width of the manifold plus the depth of the speakers makes for a pretty wide setup. I'll need to play with things and mock it up.