Would switching amps make a big difference?

+1 Almarg.

A. If Martin Logan speakers is 4ohms, .8 ohms at 20kHZ - ideal amps would have what output impedance?

B. Tube amps you might suggest?

There is a device meant for this sort of thing called the ZERO (www.zeroimpedance.com). Essentially it allows the amp to operate at a much higher load impedance while driving lower impedance loads. Its an autoformer, which is like a transformer but only one winding.

Because it only has to deal with a fairly low impedance translation, it actually has wider bandwidth than any tube amp made, even ours (which are full power from 2Hz to 100KHz or more) as it can go from about 2Hz to over 1MHz!

So if you seem to have enough power, you might try a pair of these between your amps and the speakers.

@cdc2

Excellent suggestion by Ralph (Atmasphere).

In the absence of a Zero autoformer, though, I would just say that the amplifier output impedance should be a good deal lower than the numbers I cited for the Prologue Seven, which btw correspond to a damping factor of approximately 1. Beyond that the sonics of the specific amp, and how they would synergize with the sonics of the speaker, would be the predominant consideration.

Very often amplifier output impedances are not specified (although if Stereophile has reviewed the amp the measurements section of the review will provide those numbers), but damping factors often are specified. And as you may be aware damping factor is inversely proportional to output impedance, and corresponds at least theoretically to the load impedance for which a tap is designated divided by the output impedance of that tap. So the output impedance of an 8 ohm tap should approximately correspond to 8 ohms divided by the damping factor; the output impedance of a 4 ohm tap should approximately correspond to 4 ohms divided by the damping factor, etc.

I suspect that a damping factor of at least around 8 or so would be preferable with your speakers, if a Zero is not used.

Manufacturers of tube amps having relatively high damping factors/relatively low output impedances include Music Reference, Audio Research, and McIntosh. I would be careful about preamp-to-amp impedance matching, though. I couldn’t find an output impedance spec for your preamp, but the Music Reference RM-200, which I suspect would be a fine match in other respects, has an input impedance of 30K balanced and 15K unbalanced. That is too low to be optimal with some tube preamps. Contact the preamp manufacturer if in doubt about that.

Good luck. Regards,
-- Al

Any owners of Martin Logan speakers using tube amps want to weigh in?

I've heard our amps in several customer's homes using our amps. But all of them were also using the ZEROs. No problem playing the highs at all.