Anyone listen to Zu Audio's Definition Mk3?

You reap as you sow Mr Anderson.

Did you ever have the Defs on Sistrum or just Equarack and Stillpoints?

A little info that may explain the assertion that the 4s with OTL sound possibly lean thru the midbass, and analytical with emphasised treble, versus SET 845 which may present more body and a richer tonality.
It seems that the capacitor, a Mundorf, on the high pass filter to the Radian tweeter would present a very hot treble thru the Radian 850 with OTL.
A solution to this would be the replacement of this capacitor taming the very top end a little to accommodate the OTL more successfully.
Apparently this is a rel simple job involving getting to the back of the tweeter, the capacitor is not even soldered, just a straight swap.
Interestingly on the 6Moons preview of the Druid V, the Italian distributor has done just this proceedure to alter it's high end character.

Spirit,
I reccently replaced my speaker`s only pair of caps(tweeter)with the Duelund CAST and they are simply remarkable! Keep in mind an upgrade crossover capacitor will benefit 'all' good amplifiers(you`ll hear more of what it has to offer).What is your replacement capacitor?
Regards,

Charles, not sure. All I know is that Simon the UK distributor who is a big fan of Atma Sphere certainly agreed that the hint of leaness with these OTLs could be effectively countered by a cap slightly holding back the top end and accommodating the OTL more sweetly.
I believe the 4s use Mundorf, but this isn't my field of expertise, so will leave to Phil to contribute if he wants to.
Personally, the sound of the 4s is changing for the better every day. They're really chameleon like, absolutely reflecting what's on the recording, not imposing themselves, and presenting a real open window to performance.
My Hovlands are sounding more and more like they're keepers with the 4s, but I'll keep an open mind re amp choices.

Spirit,
It`s very possible your current amplifier may be hard to beat(based on your taste,you`ve enjoyed them for some time now).The capacitor change may just be the missing factor to bring it all together.Time will tell.Get the best capacitor you can afford,it`s worth it in a system of this caliber.
Regards,