I think it likely the Pass amp would make a profound impact on your system in a very positive way. It would increase the midrange and musicality by a tremendous amount. Definitely worth the test.
Also, in general the sound character will be almost the same between the recommended amp and the demo amp… although the one recommended might be a tad less slam with a very slightly attenuated soundstage (likely to be very small difference).
Is it worth $300? I suspect it will be. I would guess that if you buy it he should refund the $300. Do you want to be sleping a 90 lb amp around. What does your system cost $30K+? The tax on my speaker wires was nearly $300.