There is another way to combine many UGen's into two channels for stereo output: rather than sending the Array's to the Mix class, combine them into a two-element Array.

{
   [
      [ SinOsc.ar(440, 0, 0.1), SinOsc.ar( 880, 0, 0.1 ), SinOsc.ar( 1660, 0, 0.1 ) ],
      [ SinOsc.ar(440, 0, 0.1), SinOsc.ar( 880, 0, 0.1 ), SinOsc.ar( 1660, 0, 0.1 ) ]
  ];
}.play;

Here, a two-element Array is the result of a Function, which gets sent the "play" message. Each of the elements is an equivalent, three-element Array where each element is a UGen.

This representation also offers another benefit: each UGen can have a different "mul" value, which will be preserved.

{
   [
      [ SinOsc.ar(440, 0, 0.2), SinOsc.ar( 880, 0, 0.1 ), SinOsc.ar( 1660, 0, 0.05 ) ],
      [ SinOsc.ar(440, 0, 0.2), SinOsc.ar( 880, 0, 0.1 ), SinOsc.ar( 1660, 0, 0.05 ) ]
  ];
}.play;

This sounds much less harsh than the first example. Try it with the Mix Class. Even with the different "mul" values, it sounds the same as the first example! This helps Mix to ensure that the total level doesn't exceed 1.0, but it has the disadvantage that careful level-balancing on your part will be erased.