I had a similar combo to Chrisrn as my 200.5 shipment was delayed. I first had a Ref 50 that I was running single-ended into my Denon 3300 for an amp. The difference in sound from the different preamp section through the Denon was pretty amazing. Much better clarity from the start that went from slightly harsh at the beginning to mellow clarity after a week or so. When the 200.5 finally arrived, the sound really settled down and opened up.
If I could only buy one at a time, it would be a tough decision, but I'd probably get the Ref 50 first just because the price just dropped and it's a lot cheaper than the 200.5. I'm not sure if a drop in price is forthcoming on the 200.5.
Hope this helps.