I think everything is a variable so you have to suck it and see how it works in your system. IME higher current has always worked better in SE OP stages of any kind including 300B but I always stay well within the limits. The sound is fuller than at higher current and perhaps it's just that the thinner sound of lower current is providing a better balance with the rest of the system which may be perceived as producing greater detail.
In my book higher HT voltages generally provide greater opportunity to design a more linear stage, especially where the plate load is a simple R. However there is a limit in OP stages with an OP TX, which are another thing again. While a higher impedance will always be more linear the trade-off may come in the form of additional leakage C caused by more windings and resulting in lower BW. It costs considerably more to alleviate this problem.
I have never found less linear operating points producing greater distortion to sound better UNLESS the particular stage is cancelling some distortion up or downstream. It's the total that counts. And, I'm not sure what the definition of "strained" really means. Is it describing noticeably higher distortion or merely excessive weight?
So back to your suggested operating points ... 400 - 450V is fine for a 300B but personally, I'd favour 60 - 70 mA, the higher the voltage the lower the current. Distortion jumps appreciably at currents that are too low and to my mind 50mA is bordering that point. If you really want to idle the tube at low power consider lower plate voltage with higher current, it's more linear.
Naz
