The Advent 300 was designed as a quality tuner/preamp, with a modest power section to get you by until you could afford a separate amp equal to its abilities. I remember reading that in an interview with Henry Kloss back when it came out.
$ensible Sound reviewed it when it first cam out (mid-'70s when they still did subjective evaluations), and compared it to the Mark Levinson of they day. They found that when they modded it by bypassing the tone controls, it came crazy-close to the sound quality of the Levinson, and it took days of listening to discover a miniscule difference in transparency in favor of the Levinson.