Oh that looks so sweet! I think that's one of the coolest looking box/speaker setups I've seen. It reminds me of the new iMac Harmon/Kardon speakers.
As to the difference in volume making a difference in the sound, I agree that the speaker volume itself will probably make it pretty close to your target of 0.6. What the increased volume will do is make your low frequency cutoff a bit lower, but decrease the overall sensitivity of the speaker. Honestly, the changes will be so subtle that you will not be able to tell the differnece. If you're really worried, get a speaker design program and run it with both boxes and see the effects.
As to filler, that will make an audible difference. Filler is a box parameter just like box volume, port size, dimensions, etc., so you can't just exclude it. The filler helps to dampen standing waves and resonances that occur in the box. Unfortunately, because of the shape of your box, you might be in trouble withresonances because of the square shape. Waves travel from the rear of the speaker, hit the perfectly parallel wall of the rear of the box, and then relfect back towards the speaker, which reflects back, and on and on. Using some filler will help with these. These resonances will make your speaker sound louder at certain frequencies and give a very uneven response.
To keep your speaker looking pretty, what I would do is buy some bumpy black foam (like the stuff they put in those big aluminum photagraphy cases) and put a thick piece on the bottom side of the box. That way it will still look good, you can still see through the other 5 sides, and you will have some deadening.
