@tuberist
Auditioning your amp against another without your preamp, front end, and ESPECIALLY with different speakers, will perhaps allow you to establish an amplifier preference. But it's highly questionable whether that preference will be applicable back at your home and with your system.
If the dealer in question is familiar you, they should allow a home trial. If otherwise, perhaps the dealer may let you take the amp home with some sort of payment/deposit for the expressed purpose of a X number of days audition. That shouldn't be a problem as long as the dealer is fully convinced they won't "loose" the cost of an amplifier. At any rate, good luck with your amplifier quest....