Hi,
Believe it or not I designed the darned thing. I googled the Advent 350 a week ago and came up with your question. The design work was from 1979 or so, and was indeen based on the 300. We used Tom Holman's phono preamp, I did the power amp and tuner. Another colleague did the preamp/tone-control. We were striving for a bit more muscle to drive our large Advents, the weight reflects a linear power supply that will drive 100W to the 4 ohm advents with no supply squat. Most amplifier power ratings are 'peak', which means it won't support sustained passages.
Anyway your reference to the ld 350 inspired me to sign up for this AudioKarma thing, my first try at this.
I don't know if your still around after all this time, thought I'd try anyway.
The reason so few people have heard of it is the company was bought about 1978, moved to NH about 1980. Lost most of the old hands including a lot of the speaker talent. Company folded again about 1981, was bought by a series of buyers. Speaker products went toJensen in Chicago.
Glad you like the 350, I have 2 of them.
Regards,
j