If they are that confident that it will sound better, let them get the one they recommend and bring it over. Hearing a different amp is of no value. I assume this is an expensive amp. I had a guy tell me that a tonearm would be a huge improvement for me, installed it on my turntable and said if I wasn't blown away, he'd refund my money (not store credit), He needed business, and I didn't think of the upgrade. I usually do smallish incremental improvements that I can hear.
He was right on the money! I was blown away, and all I risked was my time and aggravation of driving an hour up and back and waiting for him to remount the cartridge and put the arm on. Cartridge took longer than the arm.
Why can't you bring an amp home and install it yourself? It should take a couple minutes to plug in the cables. There are places that will offer a money back refund if you pay shipping back to them. TMR for example....