Audio level control ALC in passive circuit ?

Hi

An audio output amplifier rated 4 to 8 Ω, outputting 1 to 20 Watts (~2Vrms to ~11Vrms) depending on its program material and volume adjustment is desired to produce a signal line with all its dynamic range at not more than about ~6Vrms.

Was thinking on using double zeners sensing the audio level to drive series pass transistors self-biased bases to limit current when level exceeds 6Vrms, but would not have dynamic range, would behave as a clipper.

Any suggestions on how to attenuate all the audio dynamic range only when exceeds ~6Vrms ? If possible, with a passive circuit that would be connected to the speaker line far away and derive the 'limited' level ?

It is not a 70V installation, even when a 70V transformer properly connected there would sort of produce the desired attenuation.

Using double zeners to bias series transistors and limit the output would generate serious distortion above the clip point. Would that be a problem?

Not clear what you intend. Aren't you trying to have your cake and eat it? If you attenuate the signal above 6V rms then, by definition, you are restricting the dynamic range.

A compressor limiter circuit restricts the dynamic range that feeds the power amplifier. Look in Google for a circuit. It probably uses a Jfet.

http://thatcorp.com/THAT_IC_Products.shtml
That Corp has many ICs that are used to build limiters and compressors. They set the gain for you. They use voltage controlled amplifiers.
See application notes and data sheets.