Since they came out to provide service for you and apparently were competent, you may need their services again in the future. I wouldn’t burn the bridge but simply kindly decline their offer. Paying to hear an amp(s) that isn’t the one you would be buying seems a total waste.
Now, if in the future they do have the amp you would consider, the $300 question would need to be revisited BUT whatever audition fee you agree on would need to be deducted from the sales price if you buy.
As we age and lose some mobility we are forced to rely on others more. It is reasonable to pay for help provided but this one makes zero sense. Bringing you a 70 inch tv to see if you would like to buy a 55 inch one later benefits them a bit ($300 minus labor for their employee, one trip or two?). I don’t think it does anything for you except prime you to pay more than you want (up selling). The sales manager is probably hoping you would go ahead and buy the monoblocks so you’re doing the right thing in asking for advice. Don’t burn the bridge but don’t trust too much either.