( // This works!
/*** SYNTHDEF: REVERB ***/
SynthDef(\reverb, {
    arg in, out=0 ,
        predelay=0.1,
        revtime=1.8,
        lpf=4500,
        mix=0.15, amp = 1;
    var dry, wet, temp, sig;
    dry = In.ar(in, 2);
    temp = In.ar(in, 2);
    wet = Silent.ar(2); // <---- !!!
    temp = DelayN.ar(temp, 0, 2, predelay);
    16.do{
        temp = AllpassN.ar(temp, 0.05, {Rand(0.001, 0.05)}!2, revtime);
        temp = LPF.ar(temp, lpf);// Damping
        temp = wet+temp;
    };
    sig = XFade2.ar(dry, wet, mix*2-1, amp);
    Out.ar(out, sig);
}).add;

)
( // This results in "ERROR: XFade2 input 1 is not audio rate: 0 audio"
/*** SYNTHDEF: REVERB ***/
SynthDef(\reverb, {
    arg in, out=0 ,
        predelay=0.1,
        revtime=1.8,
        lpf=4500,
        mix=0.15, amp = 1;
    var dry, wet, temp, sig;
    dry = In.ar(in, 2);
    temp = In.ar(in, 2);
    wet = 0;// <---- !!!
    temp = DelayN.ar(temp, 0, 2, predelay);
    16.do{
        temp = AllpassN.ar(temp, 0.05, {Rand(0.001, 0.05)}!2, revtime);
        temp = LPF.ar(temp, lpf);// Damping
        temp = wet+temp;
    };
    sig = XFade2.ar(dry, wet, mix*2-1, amp);
    Out.ar(out, sig);
}).add;

)