SynthDef("a", { arg n=2;
    var a;
    a = switch( n,
        1, { LFPar.ar(800,0,0.1) },
        2, { LFPar.ar(600,0,0.1) },
    );
    Out.ar(0,
        if( a==nil, { LFPar.ar(400,0,0.1) }, a)
    )
}).send(s)

Synth("a", \n, 2)