Modern speakers and pentodes want NFB.
Speakers are tuned for high damping. Nowadays, nominally zero source impedance, or DF>>40. If they are tuned flat with high DF, and you give zero DF, the bass resonance will rise 10dB or 20dB.
Even guitar amps, where the speaker can be selected for best spectral balance on a hi-Z amplifier, often use about 6db NFB for DF near 1, instead of DF near 0.2 as a naked pentode gives.
Triodes give DF like 2 to 5 without any extra NFB. Rise at bass resonance is only like 3dB. While not "perfect", it usually vanishes in real-room bass nodes.
Ultralinear splits the difference.
Yes, you can enjoy a speaker without good damping, especially if you design the speaker that way. Some of the great consoles of the past ran very low damping with speakers and cabinets adjusted for pleasant bass response.
The problem with NFB and tubes is that: in many practical situations you don't have much excess gain, and you have so many coupling caps and the output transformer that you can't stablize more than 10dB-20dB NFB without crippling the amp or your wallet. That's about enough to damp a speaker, but small amounts of NFB increase IM distortion and high-order distortion.
> Solid state amps generally have a very high open loop gain, thus a lot of excess gain that can be turned into feedback, and the measured non-linearities are often very low in SS compared to tube amps.
The nonlinearity of any simple BJT stage without NFB is huge compared to any tube. But so is the gain. And you can often reduce coupling rolloffs enough to run NFB with stability. They may hide it, but it is very unusual to find a truly no-NFB BJT amp. (FETs are a lot like tubes for gain and linearity, but can often use BJT-like NFB.)
> I applied 19dB of feedback to a PP 2A3 amplifier and at the onset of output stage grid current it reduced distortion from 0.9% to 0.4%.
NFB will actually increase THD above the clipping point.
What was the change in THD at say 3dB below clipping? Just guessing: 0.5% no-NFB, <0.1% with 19dB NFB.
Of course this raises a different question: do we want THD to stay low up to a point, and then rise like a brick wall? Or a more gentle rise through and past the gross-distortion point? And of course that depends on our needs: if we never need to touch clipping, NFB will give lowest distortion in our operating zone; if we like to crank it up until it comes out bent, the gentle rise and low slope of no-NFB may be less obnoxious. (In college, with low efficiency speakers, I cranked the heck out of a 25 watt amp to get 50 watt sound. Decades later at home, I built a similar low-NF amp of 40 watts but with a high efficiency speaker, and just never felt inspired to crank it past the limit.)