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http://sw1xad.co.uk/products/?prod_cat=digital-to-analogue-converters-dacs They use vacuum tubes. Here is DAC internals video:

Have you tried this amp with low impedance headphones? https://www.bursonaudio.com/products/conductor-3/ I bought C3 but I found out that it doesn't work well with my Focal Stellia cans. C3 produces so much background noise (hiss).