I am very familiar with Dayton drivers. They are extremely underrated. I use their subwoofer drivers. Are you going to build them yourself and at which level? It would be great if you could handle the enclosure yourself. If not the MDF kit will suffice except if you want a class act finish. That will require a lot of experience or expense, more so than if you bought the entire speaker completed. To finish 4 subwoofers I just spent $400 in sealer and acrylic lacquer. Then there is having an HVLP spray setup and an explosion proof fan. A great finishing trick for MDF enclosures is black equipment cabinet carpet!
The Drivers in the X version are excellent but you should know that the 7" "woofers" are really large midrange drivers. With a BL of 8.2 and Fs or 31 Hz they are not going to have great control at the lowest frequencies. These speakers cry out for subwoofers and with subwoofers they are capable of excellent sound and are a great value. I think the way to go with tower speakers is a two way system with subwoofers.