Tube Watts vs. Solid State Watts - Any credence?

I've heard numerous times that Tube watts are not the same as Solid State watts when it comes to amps running speakers. For example, a 70 watt tube amp provides more power than a 140 watt solid state amp. Is there any credence to this or just sales talk and misguided listeners? If so, how could this be? One reason I ask is a lot of speakers recommend 50 - 300 watts of amplification but many stores have 35 watt tube amps or 50 watts tube amps running them. More power is usually better to run speakers, so why am I always hearing this stuff about a tube watt is greater than a solid state watt?
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.
@Kijanki ... thanks for your informative response. You wrote:

"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."

In my case, I drive my speakers, which have roller coaster impedance and phase angle plots, off the 4 ohm taps. To my ears, those taps sound "overall" the best. While it may result in non-optimal impedance matching at higher frequencies, I surmise that the power demands place on the amp are not that significant at such frequencies. Further, as mentioned above, the amp has a very robust power supply. Hence, I doubt that the mismatch is resulting in considerable distortion.

Kinjank, you also write that:

"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 surmised the same as posted above. I wrote that ARC said the use of NFB is part of an overall engineering solution that balances many variables.

Thanks again.
Wpc is wpc. I think there are other factors at work here to account for differences (as Mapman implies).
Beavis ... there are ... many other factors. As Kijanki posted, music is more than listening to sine waves.
10-10-15: Bifwynne
Thanks Bombaywalla ... but what is the purpose of partial cathode coupling. Is it a form of local negative feedback?? Does it ameliorate some of the adverse affects associated with NFB that Ralph has written about, e.g., TIM distortion that raises the level of odd ordered harmonics??

ARC has used this type of topology for many years.

Search & you shall find!!! (this is note to self)
Look what I found:
* an article in Stereophile where the late William Z Johnson was interviewed by John Atkinson. WZJ talks a wee bit about partial cathode coupling & gives the credit to QUAD as the initial inventor since QUAD used it in their Quad30 amp some 30 yrs prior to that interview. See para #3 from the top:

* next, I found William Zane Johnson's patent application on the partial cathode coupling (must have been an enhancement of the Quad's version?). This is publication US3566236 A that was filed in 1968 & published in 1971. Yeah, you are correct - ARC has been using this for a long time in all their products.

You can read this stuff but here is the crux (cut & pasted from the patent application)
"Still another object of the invention is to provide an improved amplifier output stage coupling with partial cathode coupling, while maintaining classic tetrode operating parameters with substantially the same efficiency and drive requirements."

hope this provides some more insight, Bifwynne.