hi nico.  when i first got my csp2 and my zen c+ i also became aware that rectifier tubes were very important in influencing what we finally hear.  so i experimented with that.  6 bendix 6101.  4 gz32. 4 chinese 274b. 2 5v4.  the mullard gz32 are indeed what i have chosen as my preferred rectifier tubes.  i have heard that gz34 tubes are even better but i cannot afford them due to their high price.  the price of the gz32 can vary widely.  shop carefully.  the bendix 6106 are practically immortal and can be gotten from tube depot new  for $50.  the chinese 274b can be gotten for $15.  in my opinion russian tubes can be put in the bottom of the box.  except for the 6nip and the 6p15p tubes that steve's zen was designed around.  much to be learned in spite of what some members of this forum say about "exotic tubes".  don pettit,  long beach, washington, usa

Thanks Don and Pale Rider for your comments. I'll give the Mullard GZ32 that I was offered local a go. Looking at the list of tubes you've already tried out Don, I'm happy to get this backup, since it saves me going through the same list to get to the same result....
Thanks!
Nico