Unless you are running 50 VSTi at the same time you wont really benefit that much from all those cores. Considering the cost of those 3970X and 3990X i would consider a Ryzen 9 instead. Well, that is unless you are also doing a lot of video rendering or other tasks that are heavily multithreaded.
DAW usage for the average user will actually benefit more from higher clockspeed. The 3970X 4,5 GHz boost clock is only when you are only using one of the cores and I’ll bet you that it is easier to overclock the Ryzen 9 to have the max boost clock on all cores than it is on a Threadripper.