The sound is somewhat different, but I like both. Maybe this is a big dilemma*scratch_one-s_head* . Yamaha overall is not so warm like Marantz, but is very fast, clean. Yamaha AX-900 is a vintage monster(17Kilo), one of the top end from Yamaha, have a lot of features, very well built, but the same time Marantz PM-66SE KI, even is only 7 kilo, and not so features(no tone controls!) is one of the Marantz top audiophille amplifier. And the sound even not so accurate like Yamaha, seem to be more musical, balanced and lot of sweet details is here present.
BTW, I know how your PM 8005 is another good amplifier from Marantz, a good return for a big sound and musicality- a historic features of Marantz house. Pleasant listening!