Hello Jan,
Driving a 200K load and 20K load will be roughly equivalent to driving an 18K load, which is still perfectly reasonable for the BeePre's balanced output.
I don't believe you'll have any issues with 108dB sensitive speakers. Generally the issues that we have are oversensitive amplifiers that need to have their inputs padded down a bit.
In the BeePre, the 300B's are operated way, way below their maximum plate dissipation, and one would expect a very long life out of good quality pair of 300B's. If I had to speculate the lifetime in hours, I would guess 20,000-50,000 hours, but that's a WAG since the BeePre hasn't even existed that long