As someone who has owned a few Harbeth's, 40's, SHL5 and SHL5+ and a few Vandy's, 5's, Quatro's (cloth) and currently Treo CT's I can understand your curiousty about the Quatro's. I have moved back and forth between these two brands for the last 25 years. I think the Quatro CT is the sweet spot in the Vandersteen lineup. Your long term happiness will really depend on your listening priorities. With the right equipment both of those speakers sound great.
My decision to move back to Vandersteen was due to the fact that all of the updates to the Harbeth's created more accurate speakers that became less enjoyable to listen to. Just my opinion. The Harbeth's still are the best speakers I have heard for low volume listening. For a long time my Vandersteen's needed some volume to sound right. It seems that the CT tweeter has fixed that and my Treo's sound great at any volume. When I had my Harbeth 40's I used a Pass 250.5 amplifier and Audio Research LS26 to power them and it was a great combination.
I might suggest reaching out to JohnnyR at the Audio Connection in Verona, NJ who is a dealer for both and who sold me all of the above mentioned speakers. He can offer some insight on both speakers. Good Luck!
My decision to move back to Vandersteen was due to the fact that all of the updates to the Harbeth's created more accurate speakers that became less enjoyable to listen to. Just my opinion. The Harbeth's still are the best speakers I have heard for low volume listening. For a long time my Vandersteen's needed some volume to sound right. It seems that the CT tweeter has fixed that and my Treo's sound great at any volume. When I had my Harbeth 40's I used a Pass 250.5 amplifier and Audio Research LS26 to power them and it was a great combination.
I might suggest reaching out to JohnnyR at the Audio Connection in Verona, NJ who is a dealer for both and who sold me all of the above mentioned speakers. He can offer some insight on both speakers. Good Luck!