Over the years, with much experimenting and listening, I have found that a very major part of amplifier sound quality is directly due to the input coupling capacitors used. This is equally true for the output coupling capacitors of the preamp. Certainly the designer of the circuitry must do the job properly, but given that this is accomplished, and given that the amplifier has adequate power and power-supply stability to do the job at hand, the purity of the sound is very dependent on the coupling capacitors used.
I have upgraded the input capacitors over 20 years on my Audire Forte 200w amplifier from 1)Black gate electrolytics (original), to 2)MIT Multicaps, then 3)TRT Infinicaps, then most recently to 4)TRT Dynamicaps.
Each of these changes was an unmistakable sonic improvement, but the Dynamicaps are a true breakthrough in dynamics and incredibly pure sound; they were a major breakthrough, rather than just a noticeable upgrade.
I have also auditioned many other amplifiers I could afford (under $4000) in my home over the years and found them disappointing (after they received great reviews in Stereophile or other magazines), and not equal in sound quality to my Audire; so I still have it.
So-why do you NEVER see the type of coupling caps employed ever given the most cursory mention in magazine reviews of amplifiers or preamplifiers?? In my opinion, many things that are FAR less significant are brought out in detail in reviews, so why no mention of this critical subject?
To give this some perspective, from 1993 to 2005 I used an Audio Research LS2-B preamplifier in my system. It came from the factory with MIT Multicaps in the output stage. In early 2005 I purchased an LS-16 Mark II, which was a huge improvement for my system over the LS2-B. I noticed that it had the TRT Infinicaps in it but, but of a physical type different from Infinicaps I had seen before (white body with Green epoxy ends, as opposed to the silver body on the ones in my amplifier since 1999). Perhaps that was the last phase of product evolution before they changed the name to Dynamicaps; perhaps the difference there is minimal? That is an open question...does anyone know about that?
But the point is, the LS-16 Mark II WAS a huge upgrade in dynamics, sonic purity, bass drum definition, clarity of massed voices and soloists...everything!
I decided to do an experiment with the LS2-B, which I still had; I upgraded its output caps from the MITs to the latest Dynamicaps. The improvement did not quite bring it up to the sonic levels of the LS-16, but I would say that 60-70% of the difference between the two preamps was instantly erased. It was a big deal, indeed. Now it did cost over $150 for the capacitors, which for a product designer is a lot of bucks to put into a product, but man what a difference!!
I am glad I have the LS-16, because it is quite amazing, but had I known about the improvement the Dynamicaps would make in the LS2-B...I would certainly have upgraded it years ago (They came out in 2003). And $150 for the parts is sure a lot less than $4000 for the new preamp!!
Well, anyway, I think audio enthusiasts(and especially audio reviewers) need to consider this subject to a greater degree than currently seems to be the case for most people.