Look an F25 is going to be very similar to the sub 2 max output at all frequencies. So two F25 would be like dual sub 2. Going off Rythmik data, quad F15HP will have +2db over dual F25. An F25 is the same as two F15 in one enclosure. Do we understand this?
I am done for now and will not comment on any more subwoofer comparisons. It on you at this point to make a decision, stick with it, and follow through.
I would like to know how the subs are when you get them. Since you saved all that dough, do yourself a solid and get some measurement equipment. It is essential if you want the best results. Many here willing to help out with measuring and any issue found with system once it can be properly diagnosed.