Solid state amplifiers and sound stage, especially front to back "depth"

@georgehifi I think I read that comment of yours from another thread a while back and have done all possible to eliminate height between speakers.

I will play with positioning as per the suggestion. That helps the practical problem. The page from @twoleftears from Pass may resolve the theoretical difference between voicing (McGowan) or no voicing (Van Alstine).

Viz., Pass: "Of course the concept of a sweet spot depends on having an idea what of what constitutes the best performance. It could be that you want the lowest measured distortion, a particular mix or phase of harmonics in the distortion waveform, the greatest efficiency, greatest power or simply the best subjective experience when you listen to it. The sweet spot is whatever you want – after all, you are the designer....If you have a distortion analyzer, you could simply run through the range of combination of values and select the result you like best. If you simply want good measurements, you might be able to stop there. If you are looking for better subjective performance, you might find this a good place to start your listening."

Your correct this is the biggest killer for depth, there should be nothing between the speakers as far back as possible, even the wall should go, and leave small 1mt sections behind the speakers for bass loading. My setup, and I have depth of image out into the garden https://ibb.co/VTRJ0Wh

Too simplistic and too drasctic in a way and not enough drastic in another way....

My speakers are near the wall and even one is few inches included in a corner wall, on my desk, with dac amplifier and computer screen between speakers...

Guess what?

my depth imaging is very good in TWO listening positions, and my sound fill my room...

 Why?

 Not only passive material treatment with a good balance between reflecting, absorbing and diffusive surface but ALSO a grid of Helmholtz resonators to activate my room and tune it....

 

As for resonance and vibration, I've neverheard that "tube amps are more resistant to this factor." I always thought they were LESS resistant, because of the glass tubes. That's a startling claim and I have never heard that before.

I have never heard this from anyone else either, just fits with logic, to me, simpler circuits, less components, heavy transformers, mass damping.  Think about it, less stuff to resonate in electrical fields means less hash i.e subtle information.  The vibrations I refer to are extremely tiny.

I had similar issues with depth and have been looking at this subject lately in my solid state system.  I can tell you the changes as a result have been simply astonishing.  Not a plug, but Critical Mass Systems footers for electronics and speakers have turned my world upside down.  From cones and sorbothane in the early days to springs to v v expensive bearing based vibration control devices now, nothing compares. 

Best wishes

Aubreybobb,
http://www.criticalmasssystems.com/productPages/centerStage.html

They look similar to the Isoacoustics design. Can you provide pricing ?

Tube amplifier are very vulnerable to vibrations for sure....

In a bad acoustical room (most room) they will fare better than most S.S. because of their more organic quality...

But many S.S. amplifier can sound organic too.... I own one...

 Vibrations is a problem for all gear,  anyway....like acoustic is or electrical noise floor....