Solid states more accurate than tubes?

The OP is slapping paint with a 12 inch wide brush. 

Yes, ... most tube amps have higher output impedances that SS amps.  My amp, an ARC Ref 150 SE, has an output impedance of roughly .76 ohms off the 8 ohm taps.   By contrast, most SS amps have output impedances that are much, much lower, ... some close to zero.

And as other posters have noted, most speakers were voiced to be driven by a low output impedance SS amp.  So, if a tube amp has a high'ish output impedance, ... yes, ... a speaker that was voiced to be driven by a SS amp, will not produce a linear FR if driven by a high'ish output impedance tube amp. 

In the case of my ARC Ref 150 SE, I calculated that the FR variation between the low impedance point of my speakers (say 4 ohms in the bass region) and the high point (say 23 ohms at the 2.2K Hz mid-to-tweeter x-over point) is roughly 1.6 db. It's an Ohm's Law thing.  

But that does not end the story, ...ergo my point about the 12 inch brush.  As don_c55 notes, a particular tube amp/speaker combo may just sound wonderful.  Just because ...

The OP should check Ralph Karsten's (Atmasphere's) posts over the years.  He speaks at great length about why some tube amps sound so good, despite being "tone controls."

Just my humble opinion.

BIF

To mahgister - 

You post that "the real accuracy of a S.S. design or tubes design is not in the subjective impression of clarity or details, it is in the perception of the real tonal timbre of instrument and voices..... 

"Is the piano sounding like a real piano?"

What you post may be true in theory, but here again you are touching on whether one's system, ... from source signal input, starting with phono cartridge, CDP or whatever, is changed as the signal works its way through the electronics, and then is converted back into sound through the speakers.

I touched on the speaker point in an earlier post.  Let's assume that the electronics and the speakers are doing a good job of converting the electronic signal back into sound energy.  What about room affects?

One more point about speakers.  There have been a number of threads which discussed at great length time coherence.  Some speaker manufacturers have designed their speakers to be purportedly time coherent.  For example, Vandersteen, Green Mountain, and possibly Thiels.  Indeed, the late, great Roy Johnson of Green Mountain wrote a number of fascinating articles about time coherence and how high order cross-overs screw up the time coordination of the various drivers in a dynamic speaker system. 

According to time coherence enthusiasts, speakers that are not time coherent can affect whether a "piano sound[s] like a real piano."

So we end where we began.  Absolutely accurate and pure system tonality is an aspirational sonic utopia that can be attempted but is rarely achieved.  So, I say, if it sounds good, enjoy the fantasy.

Just my humble opinion.

BIF