Can a Amp be "timeless" and compete with todays amps?

A listener would have to compare them in a direct A/B setting.

i think so also...

At the end acoustic/psycho-acoustic experience in a room decide what will be our choices, because nevermind the theoretical level of improvement, no ears and no room are exactly the same ...

The law of hearing are the same but the ears differ...The needs of people differ too...

And if technoloy improve always with time, there is a plateau of maturity where new technological improvement rival other different technological  advantages improvement with different trade-off...

Well this is the only modern amp that I like better than what I have. 

https://dandagostino.com/products/relentless-amplifier

The significant sort of modern advance is Class D amplification.  The better designs are pretty much indistinguishable, to me anyway, from other high end solid state designs.  But, where one does not need a whole lot of power, my preference is for low-powered tube amps.  I find that the better low-powered tube amps (roughly 40 watts or less)  to be more engaging and lively sounding at lower volume levels than solid state amps (you have to crank the volume up just a bit more with solid state to feel the music come alive).  I also think that such tube amps have a more natural sounding attack to the note (less of an artificial sounding "edge"). 

The amps I own include a parallel single-ended 2a3 amp (Audio Note Kageki), and a pushpull 349 amp built in Italy by Aldo D'Urso (sort of a replica of a Western Electric 133 amp using original Western Electric transformers and other vintage parts).  The parallel 2a3 amp puts out something like 6.5 watts, the 349 amp something around 5.5 watts.

My personal preference is for low-powered triode tubes (e.g., 2a3, 45) in either single-ended operation or pushpull, and certain pentode/tetrode tubes (KT-66, 6L6, 350B, 349) in pushpull operation.  I tend to dislike high powered tube amps that use multiple higher output tube types--the sound becomes hard and has a "glare" that I find a bit annoying.

I really like certain OTL amps as well.  They have an "immediacy" and sound extremely exciting and dynamic compared to most other amp types.  Some are operationally fussy (not the Atmasphere amps), and are a bit scary to me because I have speakers that are close to irreplaceable.  One of the best amps I've ever heard is a custom-made OTL amp.  

As for solid state amps, the ones I've heard that I particularly liked have also been low-powered, specifically the First Watt J-2 (borrowed from a friend for two weeks) and their SIT amp (heard in a friend's system).  These have a slight bit of an artificial edge to the attack of a note, but, they don't sound as dead and unengaging as most other solid state amps.  I also like some Ayre amps because they sound relatively relaxed without sounding dead; if you need more power, they are a good choice. 

When @teo_audio writes positive feedback, does he really mean feed forward?

Technically (as in: labeled for least potential in confusion), feed forward, yes.

@larryi 

Great post. I wish you could hear my Ampsandsound Nautilus which is a modern recreation of the HK Citation II. It puts out 8 watts of SE triode. The amp is overbuilt to the extreme. It is perhaps half the size of my ARC Ref 80S and yet weighs the same. I wish you could hear it not because I have any reason to believe it is any better or even as good as your low powered amps but because your impression would be interesting given your library of knowledge and amps.