Surprised? Quads are one of the all time great speakers within their limitations. They are tougher than you might think IMO. The protection circuit ought to kick in before damaging them. If you are really worried, bi amp them with a sub and they will play very loudly as it is the bass that will typically cause arcing.
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You can overdrive them if you try; I had a customer who regularly did so and damaged them several times. But they were in obivious distress before this happned. The 63s would do about 97db at one meter, I didn't sell the 988s but I believe they will go slightly louder. The size of the amp isn't as important as keeping a reasonable level. As the previous poster said within their limits they are a great speaker. Great for jazz, folk, classical at reasonable volume; not so much for rock. The protection circut is like a parachute, it will probably work but best avoid using it.
I will give you the standard power amp answer. Its always better to deliver clean unclipped power than distorted power. The Quads I understand can take only so much before frying but you listen at low levels. If you want to follow Stanwal's advice get an SPL meter and stay below 90-95db. That is pretty loud actually so it is unlikely that you will approach that and you don't need to buy a new amp. Of course it's a good excuse to get that amp you have been lusting for.
Your replies and counsel have encouraged me and I have downloaded a free Iphone SPL meter program and will self police myself and keep things at 89 db or less.
I was kind of hoping someone would encourage (enable) me to shop for a new Quad friendly amp, LOL
I guess I am more neurotic than I first thought. Keep the advice and tips coming. I don't listen to too much rock, maybe the occasional 70's British "Prog " stuff like Gentle Giant but again at more apartment like levels.
These speakers may help me save my hearing from further damage, who knows? Thanks again to all who offered their 2 cents and more.I really do appreciate it. Now what kind of subs do I need? hmmmmmm?
Open the bottom and check the flimsy sand resistor in series with the input, being paralleled by a 220uF lytic.
These are the things that would break over time and cause the speakers to distort prematurely, with a domino effect of getting the triac clamp that is activated by ionization detection to kick in and shut down the speakers.
This is a protection mechanism that is very hard on solid state amps so user-beware.
There's another thing called a thermister next to the resistor and cap that supposedly increases in resistance when the speaker is overdriven. But that thing does more harm than good IMO.
Check out other 988 989 postings on the forums on how to upgrade and watch for failure.
The 988 does not seem to need a lot of power and the best amps to drive them seem to be tube amps, but the impedance of the speaker is rather low. Quad seems to be pursuing the same course that Martin-Logan has, that of trying to make the speaker work better with transistors.
The problem is that the impedance curve of an ESL is not related to a box (so no box resonance) so the usual 'voltage source' rules that govern the behavior of most transistor amps do not apply.
Because Quad reduced the impedance down of the speaker, the use of tubes with the speaker is an issue! You solve this by getting a set of ZEROs http://www.zeroimpedance.com and then you can use a 60-150 watt tube amp quite easily.
Many ESL owners that use transistors tend to put the speaker too close to the rear wall, to deal with the fact that the transistor amp does not want to play bass on the speaker. By being close to the wall, you do get some bass reinforcement, but usually its a one-note bass. The speaker needs to be at least 5 feet from the rear wall to work right, and that is where an amplifier's difficulties in playing bass on the speaker is revealed.