I don’t necessarily agree with all the negativism here. The key, which nobody has mentioned, is his pricing. Does he sell at MSRP? if so, then he should have plenty of margin to do the demo for free.
But if he sells at a discount to MSRP, then the $300 fee plus his price is still probably less than you would pay at a dealer that sells at full MSRP and offers free auditions. So do the math. Those here that have a dealer that gives free auditions are most likely paying top dollar for their purchases.
Frankly, the second option is a much more customer friendly price system.
Jerry
PS. I would be reluctant to do the demo with a different amp. While you will be listening to less than a watt, I don't know if you can be sure that the first watt sounds the same in the 2 amps.