This video shows extracts from a test we made at Abbey Road Studio 2, where we compared two microphone preamplifiers on female vocal. Tested are the Universal Audio Solo/610, which is a vacuum tube preamp, and the Focusrite ISA One, which uses transistors.

Both preamps feature gain and output controls. In theory, using a high gain and low output should give more warmth; low gain and high output should offer a cleaner sound. Listen to the results and decide for yourself...