Sonic effect of balance/bias setting of output tubes for tubed amplifiers.

I recently had my Audio Research (ARC) Dual 51 amplifier serviced.  In passing, the technician said, "I've adjusted the AC bias for the lowest distortion."  

Out of curiosity, when I had the amp home, and after about three hours of listening, I checked the balance and bias setting for the 6550WE output tubes of the amplifier.  They were set at 46 ma. balance, 48 ma.bias.  I thought, "Is this correct?" 

In reading the instruction manual for the Dual 51, ARC says,"Trim DC balance control(s) for 55 ma. reading per output tube - readjust bias control(s) if necessary."   So I readjusted the readings to 55 ma. as recommended by ARC and listened.  Then I began to wonder if the technician was on to something with his comment, so I reset the balance/bias reading back to his settings.  

I immediately noticed the following with his settings: More delicacy, openness, detail, and transparency in the sound coming from the speakers (KLH Nines).  It was as if the rear sound-stage was flowing "through" the Nines, rather than "from" them.  There was more clarity of detail to all aspects of the sound than had been with the settings at 55 ma.  

Is there actually a "range" that produces better sound at certain balance/bias settings for the output tubes?
The short answer is 'Yes.' The long answer is a lot longer but arrives at the same place :)  Bias affects distortion, not power and for almost any amplifier there is an ideal bias setting for which the amp will produce the least distortion; depending on the amp there are often tradeoffs.
Its all tradeoffs, all the time. Not always apparent. But always there.
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“Tradeoffs?”  Like what?  Will certain musical frequencies be more prominent than others, for example? (Deeper bass, smoother midrange, etc.). I’d Like to know what to listen for.