Anyone listen to Zu Audio's Definition Mk3?

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,

Well Charles, I've just checked the main UK source of Dueland Cast PIO Copper caps, and depending on spec, they range in price from $300 to $3750. Gulp! I presume I need these for the preamp only? How many am I likely to need, just 2 with labour charge to install would cover the cost of a brand new Atma S30! Anyhow, I thought one of the big positives re Bob Hovland was that his Hov MusiCaps were great performers and sought after by other component designers.
Isn't this all a bit of a punt? I mean there's no way to a-b an upgraded amp/spkrs with caps installed against the unadulterated originals. This is almost as risky as a particular cartridge dealer in the UK refusing home trial, expecting me to buy without being totally secure in hearing it in my system.
Lack of comparison leaves me a little anxious re the outcome. I have to emphasise the Hovs only demonstrate a hint of glassiness on some material, and I'm still convinced this is eminently tolerable with the liquid neutrality I hear most of the time. Additionally, the 4s are changing for the better literally day to day, bringing much needed warmth to offset occas coolness. Still convinced careful cable/power cord choice (Zu Event), vibration control (settling on Symposium Isis and prob Minus K on wall shelf for tt) and balanced power overkill (Westwick Audio 8kVA transformer) will maximise neutrality with just the right amount of warmth to hopefully fully balance the final sound. But I'm not going to totally dismiss possible SET/OTL avenues.

Spirit,
You make some very good points,there`re different pathways to upgrading a system. The cost of the Duelund CAST is based on the capacitor size/value in uf. Mine are 5.6uF(speaker) and cost me 850 USD.The 1 pair I placed in my DAC(output capacitors, 1uF) cost 440 USD. The improvement is 'very' substantial. Spirit, depending on the size of your speaker caps(what size are they?),the cost may be equal or even less than many high end cables(power or interconnect).Just something to think about as tweeter cap quality is a big contributor to the overall sound.

Charles, I had a little bit of a mindstorm, and thought you were suggesting I replace the caps in my Hov pre! That would have cost me a tidy sum, in effect the cost of totally new amplification!
I'll check with Sean on the cap value on the tweeters in the 4s and give this some serious consideration. Luckily here this is a rel. simple install job, no soldering needed, just temp. dismantling of tweeter.