75% depth front to back. This means that there is space behind the sub-enclosure to take advantage of that cabinet volume that is otherwise wasted on the 10" drivers. Yes, you can keep the Delta 15. Is it the A version? You will need to pick 2 identical 10" drivers. Let us know your choices and maybe there is an obvious better choice. Belgium, this means that some options that I might otherwise suggest are not easily (or affordably) available to you.