As an update here, I reached out to the US distributor of Mastersound, who reached out to Mastersound directly to get some clarification about my concerns on the 15wpc rating. Answers are below.
1. In speaking with MastersounD, they have given a spec at the “highest” voltage in electricity. For example, in the UK market, voltage is 220v, but in many cases, homes will have 230v or even 240v. In a more normal voltage like 220v, their power is more like 10-11 wpc. This Is still higher than what others are getting out of a pair of 300b tubes, but that is due to the in-house built transformers. These alone have been getting approx 30-40% more output of a tube as they are achieving 95% of the efficiency of the tube. The common characteristic sound of our amps are that they have more energy and dynamics than our competitors amps.
2. MastersounD regulates the 300b at a very “quiet” value without risks to the life of the tube. The 300b tube is a very delicate tube and especially during transport. That being said, they have many customers who are still on their original tubes after 20 years. It all depends on the quality of the tube.