We tried both 15 wpc 300B and 16 wpc 211 SET amplifiers with Merlin VSM-ME, in my buddy Spencer's (Sbank) system, Pubul57.
The 300B was a parallel single-ended amplifier, with 2 TJ mesh plate 300B output tubes. The 211 featured a single output tube. I have my 211 amp modded to run 845 tubes as well, but we didn't get to it that day.
You'll be able to drive the Merlins like crazy with any of these tubes, as we did that day. In my opinion, the midrange/treble of the 300B cannot be approached by a 211 (or 845, especially) amplifier. However, there is a lack of low-end with the 300B amp in comparison to the 211 (and, again, especially, the 845 which has tons and tons of bass) that may leave some audiophiles wanting.
For me, it's a closer race as to which I prefer, but Spencer (hopefully, he'll chime in here) was most emphatic in his feeling that the 211 was the way to go because of the paucity of low frequency performance via the 300B. Again, you can listen at very high volumes with either amp, and that is probably where the two of us will agree most.
DISCLAIMER: I am the North American importer/distributor for Opera Audio/Consonance high-end audio components - which the aforementioned amplifiers were