Could you expand on that? I'm definitely not saying that doing your own crossover is easy. Properly correcting and time/phase aligning with REW+MiniDSP is a bunch of work and requires reading and research especially if you don't already have a basic understanding. If you have the money to spare, GLM will certainly do a great job which is not guaranteed by DIY.
On the other hand, Genelec sub pricing is truly ridiculous, for the price of the single 7360A you could buy two Rythmik F12s and have money left over for software and hardware to do multi-sub optimization.
As far as I understand, GLM is not that sophisticated in terms of bass management.
In GLM 4.0 which is the upcoming(? not sure if out yet) version, it will do automatic sequential frequency response and phase correction of multiple subwoofers. But it won't combine and independently equalize them to produce a better sum across the listening positions the way the Dirac Live bass module does, or the way you can do using Multi-Sub Optimizer manually.
If GLM corrected the sum bass response at the listening position by using every available woofer(sub or otherwise) then I would agree you can't possibly do better, but I don't think it is capable of that. Given the price of their subs, it really should be.