800L (28 cubic feet) net is probably too large for those subs. Very few 15's will do well in greater than 5 cubic feet per sub and your frequency response may get peaky as you increase volume too much. "Space makes bass" is very true and larger box almost always = louder and more efficient but you will hit a point of diminishing returns where adding more space gives you less and less return in output gains.
Download WInISD (it is free) and you can plug in your subs TS parameters and play around with modeling size/tuning and see predicted performance.
As you have discovered, big box and low tuning will both require a very large port. You will probably have to sacrifice something either tune a little higher or go with less port area. A safe rule of thumb commonly used is to allow 15 square inches of port area per cubic foot of net box volume. You will need to convert this to liters and cm. 15" per cube has been very reliable in my experience, going up to or even over 20" per cube may be acceptable in SPL/burp only box, and you probably won't get into too much trouble until you get below 10" per cube if you have to shrink your port a little bit to fit it all in the box.