845/211 tube amplifiers with kef reference 5?

Ben,
I am not sure if Kara is still making amps, but I have the utmost respect for http://www.dehavillandhifi.com/. They specialized in your wheelhouse and when I had the right speakers, their 845s really sang! They played around with some other tubes GM70 or something like that, trying to keep that magic with higher outputs, but I haven't heard those...I guess they never found much audience, but perhaps others with firsthand experience could chime in about that approach for your needs. 
Yes, the 6C33 are a good option to consider and quite a robust tube. Not the bargain of the 6AS7s used in the Atma-Spheres, would you should also read up on. They have sold a ton of amps for a reason! Cheers,
Spencer

Those Kefs like power.  I think you would love the sound but hate bass performance.  I had run Art Audio Quartets which are 45w 845 monos for almost 18 months.  I am the importer.  They are  Incredible amps but they would not handle bass at reference levels and I would not recommend them with those speakers.  
Maybe bi-amp.  An 845 for mids and treble and a 200w SS for the bass?

The is probably above budget but mention it anyway.  For an alternate pure tube amp, you would love the sound of the KR Kronzilla.  It is a much more powerful tube but pricing stings a bit.  The list for the Kronzilla DXL monos is $44k but they are truly amazing amps and are 140w. They don’t come up used often but they could be had for $15k to $20k used.  Might be an Interesting option.  

@thegreenman  Unless you are in a small room there is simply now way any SET is going to do that speaker justice and the other way 'round.


Don't do it! Your speakers need a bit of power, being rated at 90dB. KEF rates the speaker at 8 ohms, but the fact that it employs dual woofers in parallel and a look at the impedance curve (published by Stereophile) shows that in the bass region, its really a 4 ohm load. So in that region, its actual efficiency is more like 87dB rather than 90 (Sensitivity is 2.83volts at one meter; converting to efficiency, which is a more useful spec for tube amps, you get 2 watts into 4 ohms, subtract 3dB to get back to 1 watt, you get 87dB)! So a tube amp with no feedback (like most SETs for example) will be incompatible with this speaker; even if the amp made enough power (which no practical SET does). On this speaker, it would be bass shy.


The speaker expects the amp to behave as a voltage source (putting out constant voltage regardless of load) but SETs being tube amps with no feedback behave as a power source (they try to put out constant power regardless of load). If you want to know more about this see
http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html
If you want to put a tube amp on this speaker, get one with some power; I would recommend 60 watts as a minimum. Run it on the 4 ohm tap. Keep the speaker cables short (monoblocks are helpful in this regard).

Bel Canto SET80 use 845R power triodes and output 70Watts per channel.

https://www.stereophile.com/tubepoweramps/598bel/index.html

Bel Canto SET80 use 845R power triodes and output 70Watts per channel.

This is why this amp won't work with the KEF:
http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html
The Bel Canto is a Power Paradigm device; the speaker is a Voltage Paradigm device. Anytime the two technologies are mixed, you run the risk of a tonal anomaly. In this case, it will be bass shyness.