Boy, that’s a tough one. Honestly, the magnitude of difference between the BHK pre and the PMG pre is the size of the Grand Canyon, while the MKII to 512 difference is crazy big but not Grand Canyon sized.
Without question biggest bang for the buck is upgrading the preamplifier. I cannot adequately express how big the difference is. The barriers and fences put up by the BHK pre were never noticeable to me—it just sounds amazing—until you hear them removed. Hard to put into words but some sonic barriers never reveal themselves until you hear them removed, which is what happens when you put in the PMG preamp. Holy f*ck!