Ok, here goes my reply. I can't comment on your electronics, I haven't heard them, but if the phono stage is a weak point, I'd have to agree that it and the arm cable maybe the bigger areas to hit first.
Also, on your SME 20/2a, I've had big results with isolation. Currently I'm using a Symposium Ultra shelf sitting on a Arcici suspense stand. I have a Finite Elemente Pagode Master Reference stand on order, I'll see if that improves things. Power cords on the motor controller also help. I've used several and settled on the Omega Micro via Walker.
Arm cables, I've written a review on the Purist Venestas I believe. I've also used the Graham IC 50 and Hovland Music groove cables. These make a marked improvement.
As for cartridges, I've stayed in the lyra line over the years. Started with a Lydian Beta, then a Helicon, now a Skala. The Skala is killer. I don't know about the others but know that they seem to be good matches with the SME 20/2 due to other posts around here. I have a review posted on the Skala as well that might help you some. The lyra's all output about .5 so that helps you some. Plus, I doubt they'd get confused any-especially the Helicon or Skala. Setup is very critical here.