Bobby know his speaker better than anyone, and hard to argue with EL34s, the combos I have tried with them have been superb, and always preferable to 6550 or KT88s with the VSM. Another tube that takes well to the Merlins is the EL84s, which you would find in the Manley Stingray, which is probably overprices at the new pricing scheme, or in my case, I use the Music Reference RM10s, which replaced the EL34 based RM9 Special Edition (one of the finest high powered tube amps I have owned) - the 35 watts from the RM10 seem more than enough to make them sing (in my room and SPL preferences).