Hi, After SBAF members recommended HD650, when I was looking to buy an headphone; I decided to do a listening comparison for HD650/HD600/HE560 which were on my shortlist. I also wanted to listen to HE400S but the shop didnt have the stock. I also listened to some other brands but forgot their names. So this is how it went.. The shop had a Chord Hugo as their DAC/AmP. HE560: Good, but the vocals sounded a bit thin. In the end HD600 & HD650 were the most impressive. For most of the music the two sounded very similar. Then for the vocals HD650 finally won me over. The lush and rich vocals of HD650 were clearly superior to HD600. I just loved that sound. At very low volumes or with poor amp, I think HD600 sounds better. But since I normally listen at higher volumes the HD650 was the absolute clear choice. In my mind I thanked SBAF and walked away with a brand new HD650. Okay, now to the question for experts: I got a NAD M51on a good deal and now using that as the DAC-I like it.. I want to upgrade from my Magni2 Uber and want to purchase a tube amp which matches with NAD M51. What tube amp will match with the NAD M51 & HD650?? My budget is around USD500-1000.