I'd think a bit about the Quadra + Expander + Maths all in the same rack, as Quadra + Expander will duplicate a lot of Maths functionality. They'll match really well together, I think, but unless you're looking to do complex west coast generative patches you don't really need 6 LFOs / EGs to control a DPO. Of course you could get both and sell whichever one if you don't need it though, which is the best way if you can afford it.