I just don't get it. An amplifier should just supply power to the signal without adding anything to the signal. Electronic amps are so close to perfect that you need to look elsewhere for the 'perfect' sound, especially the speakers.
If you *like* the distortion that tubes provide, there are ways to make power MOSFETs (in analog mode, not class D) sound like tubes. Much less expensive and far more power. If you are driving your amp into distortion, you need a bigger amp. IMO, your sound system should make the sound the same as the artist intended so look at speakers and the sound source.
Might be in the TLDR post category....
First of all thanks for watching this thread! I was right with you when I began my audio passion over 40 years ago and for most of those 40 years as an engineer.
From being a customer I got friendly with Paul Heath and Jim Gala (early highend dealers in Rochester) and spent alot of weekends and after hours at their places listening to many, many different components over the years. What struck me that components (amps, phono cartridges, CD players) all within the noise of each other in measured specifications all sounded different, some profoundly so.
In the case of amplifiers, some sounded glorious, some sounded sterile. Some provided spectacular 3D precision imaging, others provided a flat soundspace with no depth, some were just sloppy. Some amplifiers had me engrossed in the music, others left me just listening to the electronics. The dividing line was NOT tube versus solid state, there were excellent and poor experiences with both.
I was also a semi acquaintance of Roger Modjeski (actually he was just generous with his time on the phone with me and my buddy). In my opinion he was an engineer's engineer. He taught me it was actually easy to get ruler flat freq response and get to the limit of measurability of THD in any amplifier design. Just increase the open loop gain and provide as much negative feedback required to get the specs you want. But it will sound terrible.
Nelson Pass (Pass Labs) makes glorious solid-state sounding amplifiers that measure well and sound wonderful. His design philosophy is that it has to operate totally or mainly class A and minimize the NFB necessary by having a low distortion open-loop design. If I could afford or want to spend the $ on a Pass Labs amp this DIY thread would not exist.
As far a tube sound being due to added distortion, that may true in some cases but not in mine and that is an incorrect generalization. Below is the spice sim results of the amp I am designing for my next project. Its an UNSET push-pull using 1625 (WWII era) tubes. It will produce about 20W is class A which is more than enough for my 96db/1W speakers. In my last project the Spice sim was confirmed by measurements after completion.
This is the FFT predicting harmonic distortion. There is essentially zero 2nd order, and 3rd harmonic is over 70db down. So thats between 0.03 - 0.1% THD (The Mark Levenson No536 is rated at 0.3% THD). The AES has shown that these levels of distortion are inaudible in double blind tests.
So with the remaining years I have left and less with good hearing I will build a system that puts a broad smile on my face when listening to music (not the system). But I can also do it under a good measurement regime. So I can have my cake and eat it too. Plus tubes are cool.