Amplify sinewave to 6A
Try connecting some ordinary power transformer to the output of the audio amp.  Let's say you have a 115 V to 12 V transformer.  Connect the 115 V primary to the audio amp output.  In theory if the transformer was ideal, you would get 9.6 times the output current from what the amp is producing.  Of course the voltage would also be lower by that same factor.

Real transformers have losses, so the voltage x current out will be less than the voltage x current in.

The transformer also has to be rated for more than the output current you want.  In the case of a power transformer, the rated output current is achievable with the intended input voltage.  However, your amp won't put out that much.  If your audio amp can drive 50 W into 8 &Omega;, then it is only producing 20 V.  A power transformer specified at 115 V in may not be able to produce its full rated output current with only 20 V in, even with the secondary shorted.  Over-specifying the power transformer will help.