Tom:

you can use your 300 watt dummy load on the amp.  Just turn down the drive 
until your total output is under 300 watts.

If memory serves, it takes around 60 watts to get the full 600 out of the 
als600. so try 25  watts drive, and if you have a tw-1 put it behind the amp 
and measure the output power.  set your drive for about 250 watts out.

As to the house ground, it could be in two places, depending on the age of 
the house.

1. dropping directly from the meter base streight down to a ground rod. 
This is usually #8 or #10 bare wire. Sometimes even 4 or 6.

The other place at leaset in my old house, it ran down the wall from the 
breaker box to the cold water pipe.