Do you use one brand of electronics or are you mixing brands what has worked best for you?

Mix and match. Each component has been chosen separately. Also, the manufacturers chosen don't make all components. For example, the speaker manufacturer doesn't make a CD player. I have settled on one manufacturer for interconnects and speaker cables. Power cables are a mixture. I'm very happy with the sound of my system. I can imagine that it could be improved if all of the cables were from one manufacturer, for example. 

Brand Synergy:

It seems that there is much less brand synergy at entry level, but can increase with better parts, engineering, and accompanying costs.

It seems less brand synergy for tube gear than SS

I wonder if some brand synergy is “designed” to work better with same brand components, while other brands design components independently.  

Depends on the system.  With my main 2.2 music only setup I have Rotel separates - amp, preamp and CD player all in the same MK II series.  The rest of the setup has different brands.  My Sonos systems obviously have all Sonos components.  All of my other systems are mix and match.  I also mix and match new and used components.  A fun part of the hobby is mixing and matching.  Adding different equipment from different manufacturers over time.  That’s one of the reasons I prefer separates more often.  

... any coloration acts like a filter that accumulates or cancels out across the entire signal chain, ultimately altering the system’s frequency response. To minimize that effect, it’s preferable to keep components as neutral as possible and preserve that neutrality until the very end of the chain, where you can make intentional adjustments to the timbre or overall tonal balance for better coherence.

If you agree with the aforementioned notion of keeping the source neutral and tuning the tonal balance at the back of the signal chain, what are the most effective ways to add warmth or smoothness to the system? You could rank them as well and please provide examples?  

1) Hybrid SS with tube buffer

2) Hybrid tube preamp stage with SS output stage

3) Tube preamp + SS amp

4) Tube amp

5) Class A SS amp with zero/low feedback

6) High bias Class A/AB with zero/low feedback

I mix and match, though all of my new tube gear is all Aric Audio