Thanks Ricci, I really appreciate your input. Yeah I have no faith in the output I am getting from winisd but 45l, which seems tiny, seems to top out the extension. The smaller the better for me (17cube is perfect) but if its wrong I am worries about the transient response if its too small. On the flip side I am also worried about damaging the drivers. I already have an inuke 6000 to drive 2 of them but it doesn't have a limiter (eq via minidsp). According to winisd the power for xmax drops off pretty fast below 22. I could get 8ohms to be safe but then I'll be missing out on output.
Unfortunately I am size restricted by how big I can go with the subs as its not a dedicated room otherwise I would just get 2 BMS 18N862 which are pretty much the same price (or even the 18DS). The location for the second sub just cant accommodate anything bigger than an 18 cube . I would however like to go for the best quality drivers I can to deliver the clean bass as is practically possible and plenty of output above 40 to add some slam when needed. I had considered the Alpine swr 15 and Datyons 15 HO which are cheaper (and easy to model!) but I do like the idea of pa type subs.