It’s your money, may be worth it to you. While $300 might seem excessive, relative to the cost of swapping components in and out to elevate your system (ie shipping costs) this amount seems small. Also, dealer support might be a good thing.
From the dealer’s point of view, I can understand their hesitant to loan out expensive gear for trial - the effort, time, risk of customer abusement, and cost to transport and maybe set up for free to any customer asking may not be worth it to them. They’d rather have a more serious/committed customer with a reasonable chance of a sale in which they would be compensated for their efforts, and not merely satisfying a potential customer’s curiosity.
Maybe the dealer is willing to give you $300 credit for anything else he sells if you don't like the amp. I'm a big fan of win-win.