Harbeth 40.2 or Vandersteen Quatro CT

+1 on contacting John Rutan (audioconnection).
He will give the dope on what you need to know.
Great advice, and free from pressure.
Bob

They are very different speakers.  If you value things like 3D imaging/soundstage, extended bass, and neutrality there’s a lot you’ll like about the Quatros.  But if it’s more the “sweetness” and smooth sound of the Harbeths you might feel a little left out in the cold with the Vandys.  To my ears, speakers that kind of bridge the gap a little more would be Verity Audio or Sonus Faber, or maybe even ProAc.  Best of luck in your search. 

I agree with Soix. Having owned the Vandersteen Treo CT and the Harbeth C7 and M30.1. You can’t go wrong with either, but I warn you. You may have regrets selling your Harbeth. Vandersteens do have an awesome soundstage and imaging to die for...completely disappears in the room, but there is something magical in the mid range of the Harbeths that keeps pulling me back. The Vandersteen are truly awesome for reproducing live music...best I’ve heard, but if you play a lot vocals and jazz, I think the Harbeths have an edge. Just my opinion. I would not make any decision until you’ve had a chance to audition the Vandersteens somehow.

And if you do go with the Vandersteens, the Pass Labs amps will probably hold you back from what they are truly capable of. I say that as a former owner of XA 60.8s, but I have not heard the 260.8s. A no global feedback amp is a better choice. I have the M5-HPAs which were a major improvement. Gone was upper midrange glare. Welcomed was a better sense of realism and immediacy.

I agree that the Harbeth will have a different mid-range sound.  You can probably find a used pair of Vandy 5As for close to $6500 or less at this time so that may be a great option.

Since the Quattros have built-in amps in each speaker for the subwoofers, your amps will only be driving the mids and tweeters (using the hi-pass filters) - completely false statement.  Increasing the power supply capacitors in the amplifier we build also added to the bass improvement in my 5As.

The Vandersteens do so much right and can be adjusted for your room now and in the future.  I do miss the mid-range of my Alon V MKII speakers though in direct comparison.

I also built a pair of the HP filters for my 5s, the original ones to me use basic parts which I upgraded and now they sound much better to my ears.

Happy Listening.