Based on prevailing opinion from the lands, where this tube came from, the best audio quality is achieved with transformer or choke load and at plate voltage above 200V. In a sense, second thing drives the first. Tube is very linear so no surprise the sound is typically described as 'precise' and 'clean', sometimes even 'analytical'. I guess it could be perceived as harsh if listener is used to large second harmonic in the signal. Or, which is not always clear from comments, if the power rail is not capable of delivering clean power during peaks of signal, the sagging of B+ could impact the sound quality.
I would rank 6S45P above 6(inverted)E5P in sound quality as a driver. I do not have experience with using them as output devices, which seems to be popular in headphone amplifiers.
Hi, apologies, I was talking about the 6C45P. The CCS load seems to have a more “dynamic” presentation and a more solid and better defined mid bass/bass.
Also, the 6C45P requires careful attention to RF mitigation measures, they are high GM RF amplifying devices after all. Use a stopper resistors on each grid and I also use one (100ohm) on the Anode. Ferrites on the heater wires are appropriate too.
My “feeling”, not necessarily a “fact”, is that with proper RF hygiene they are not “harsh” sounding at all.
My 2cents, ymmv etc etc