Relationship of amplifier sound to transformer quality?

Monolith Magnetics makes some very nice (and expensive) transformers. The OPTs I use in my 833C SE amps weigh 62lbs each, with amorphous double-C cores and virgin teflon interwinding insulation. Here's a picture of one during its construction.

http://i1243.photobucket.com/albums/gg559/Quagmire22/833OPTcoilandcore_zps9fb41a95.jpg

They're not cheap but high quality very rarely is.

And yes, the transformers have a huge effect on the sound of tube-based amps, preamps, DACs, whatever.

It is a direct correlation.

...quality transformer is about 80% of final sound...it is essential...

Another factor in transformer selection....

• The transformer acts like a reservoir for electrical energy.
  
• When loud, dynamic music (very fast, like drums) is encountered, the energy required to realistically reproduce that instrument is provided initially from the transformer
• A small trasformer is unable to provide all the energy and as a result there is a tiny drop in the circuits internal voltage
• This is music, the the internal voltage is always changing - not good for an amp
• The voltage fluctuations degrade the amps output signal
• The Bigger the transformer, the better the amp is able to handle the demand for electrical energy, fluctuations are minimal and the result is better quality music

Naim is one company that has always followed this approach. They even build seperate power supplies for most of their gear and it pays dividends. Many other high end amps adopt the same philosophy.

I’ve made seperate power supplies with large transformers for a few components and each time the improvements were significant - well worth the investment. And that was simply increasing the size of the transformer.

Factor in the other advantages listed above and you can see why good designers consider transformers crucial to good design.

Regards