Richard Clark $10,000 Amplifier Challenge - Why Couldn't Anyone Pass this Test??

They need about 15 Watts of the best amplification

The above quote is taken from the link Erik provided.

I've serviced the 15-watt tube amps that Quad made to go with the '57s; in a nutshell driving a 'difficult load' is something they can't/don't do. But they drive the Quads just fine.

When 15 watts is all that's needed, its not a hard load and no mistake :)

As Erik points out, the impedance curve is very typical of an ESL.

I suspect that what Shadorne calls 'difficult' (and for that matter the author at the link Erik provided) is the simple fact that solid state amps (even ones known for driving difficult loads) can't make as much power into the relatively high impedance that the Quad presents at low frequencies. So you might need a 60-watt solid state amp to make the sound pressure on the Quad that a 15-watt tube amp can. But it will still play it, and if one puts one's hand on the heatsinks of said solid state amp, it will be found that the heatsinks won't get all that warm, despite hours of operation (unless the amp is class A)!

IOW, the solid state amp isn't working that hard. Why? Its not a difficult load; the amp is never asked to make anywhere near the power of which its capable. The 'difficulty' might be that the Quad will easily reveal the shortcomings of any amp which is asked to play it; so to get the best out of this speaker, you need a good amp. But that amp does not need a lot of current capability, which is traditionally associated with driving 'difficult' loads. We have lots of customers with Quads and they usually use our smaller amps. Folks, 'small OTL' and 'the ability to drive difficult loads' are two concepts not normally found together :)

The simple fact is, this speaker can be driven by a variety of amps (including SETs) that are not known for being able to drive 'difficult' loads. I would not use an SET in this case simply because I don't know of 15-watt SETs  that can make the bandwidth that the Quads can, but if you have one, it will work fine.

This is an excellent example of how not all amps are the solution for all speakers!

The Quads have issues. First of all they need to be on stands, I had the Arcici stands. Secondly, they need to have their metal grills removed. The grills suffocate the speakers. The Mylar dust covers should also be removed. If you’re nervous about dust in the room get an air purifier or two. Finally, the Quads benefit from better power cords. All of these actions result in tremendous dynamics and bass performance.

Old news - Peter Aczel was saying this in the 80's.  Not worth talking about IMO.  I've owned $25k amp/preamp combos and liked them and I've also used a $200 Onkyo reciever while an amp was being repaired and I liked that too.  Personally, I think the more expensive high-end stuff sounded better and that's what I choose to own and listen to and I couldn't give two shites about what someone else thinks.  And respectfully, either should you .  

I cannot totally Agree,  I have been running pure class A Pass Labs 
And most certainly better then the majority of Class An amps 
Even my brothers Pass 250.8 is far superior yhen most evrn st low volumes 
We compared  a bunch of Audiophile friends gear over 6 months.
The average AB amp could not compete in resolution .
One big reason is the very High bias in class A  for almost the first 20 watts 
Which is a lot with a 90db dpeaker..   The Valvet mono blocks are around 100 wpc
Into 4 ohms pure class A and incredibly detailed at  Low volume .
Vitus, is another HIGH POWER great sounding amp even met low late night 
Low level, as well as Gryphon Diablo ?this and the Vitus,integrated are the 2 best 
Integrated amps I have ever heard at all levels and passed the acid test .
I owned a Aufio store until 06 in Europe the new models are even Better.
If you are willing to spend these are exceptions to the rule.

With respect to the talk on old and new violins, I'm surprised no one has mentioned these two studies:

Is A Stradivarius Violin Easier To Hear? Science Says Nope

Double-Blind Violin Test: Can You Pick The Strad?