I would think that the setting of the cutoff will also depend on other things like room acoustics, amp, etc.
I'm also not sure how they specify that cutoff, but I imagine it is a -3dB point.
Generally speaking, I would estimate that you would want the cutoff on the sub set somewhere near where your speakers roll off to about -6 dB or so. Any higher and I would imagine you are likely to get too much energy in the room at that frequency.
I would imagine your speakers are -3dB at 39 Hz and -6dB somewhere below that (perhaps 35 Hz). The MJ cutoff at 40Hz is probably borderline for you. However, the other factors involved may or may not make it acceptably 'disappear' in your system.
For the pricepoint, I think the MJ is most likely a steal and would be worth a shot for you to try, especially since it's about half the cost of the REL. I would suggest you find a shop that might let you demo the MJ.
I'm not sure about other subs, but at this price point, I doubt there are others that have the same musical performance as the MJ and REL. The only other that comes to mind is the Monitor Audio ASW100? Also with a cutoff at 40Hz and also cheaper than the REL. Or perhaps you can find a used Q150E or an older model like the Q100? Check the cutoff specs first though.
Chris