Vacuum tube amplifier or preamplifier?

If you could only upgrade one product in the chain to a vacuum tube for added warmth, would it be the preamp or power amp and why? If you want to learn more, grab a copy of Paul's new book, The Audiophile's Guide. https://www.psaudio.com/products/the-audiophiles-guide/

Видео Vacuum tube amplifier or preamplifier? канала PS Audio