Tube Watts vs. Solid State Watts - Any credence?

The difference in perceived loudness between 70W and 140W amplifier is only 22%. Tube amps, when overdriven tend to produce "soft clipping" (softer/gradual overdive transition), while SS amp just chops off the peaks producing a lot of unpleasant sounding odd order harmonics.

Some SS amp were designed to soft clip, including my class D Rowland 102 (and some NADs) but most of them don't. Overdriving SS amp is not only much more noticeable, but can also damage tweeters because of excessive energy in high frequency harmonics.

Music requires very little average power, unless one listens to sine waves. It is possible that two of 50W amplifiers can be designed with very different headrooms. One might have small headroom being, for instance class A amp while the oder has huge headroom with average power limited only by size of power supply and heatsinks. Amp with higher headroom might appear much louder without distortion.

14dB is rather shallow feedback.

Damping factor of 14 is OK. 8ohm speaker's impedance is mostly resistive. Assuming, that it is approx 6 ohm it limits effective DF to 1.33 . Amps DF of 14 will make it worse only by 9.5% - irrelevant.

There is a lot of local NFB in almost every amp. Any resistance in cathode is a form of NFB. Global NFB doesn't have to create TIM if it is applied within certain limits. It improves pretty much everything - bandwidth, output impedance, THD & IMD. Great sounding amp with small amount of NFB requires great design and quality components. Unfortunately it is cheaper to achieve the same using cheaper design and excessive amount of NFB hence creating overshoots (odd harmonics in frequency domain) and unpleasant bright sound.

I believe Atmasphere mentioned once particular B&W speakers consisting of lower efficiency 4 ohm bass speakers with higher efficiency 8 ohm midrange and tweeter. Speaker has great bass when amplifier doubles current on 4 ohm speaker, but not so great when driven by tube amp that doesn't do that.

Large headroom doesn't mean poor performance. It means that amp has ability to output higher momentary peaks. Otherwise it is power limited by power supply and/or heatsinks. That's what music is - peaks, gaps and very low average power (few percent of the peak). Power test is done with continuous sinewaves.

85W vs 70W 5.7% loudness increase
45W vs 40W 3.4% loudness increase