An A21 puts out about 150 watts at idle and likely about the same when driving reasonably efficient speakers at normal volumes so I 'd think an A51 should put out at least the same amount of heat or perhaps more. Sounds like the amp is dead. Sorry, don't know what kind of recourse you have with AudiogoN.
Main Kef: Reference 205/2 & 202/2c, Surrounds: Kef XQ40, Velodyne Optimum 12, Integra DHC 80.3, Ayre K-5xeMP, Oppo BDP-103, Bryston 4Bsst2, Parasound Halo A31. Second B&W: 685 (3), CCM618, Def Tech Powerfield 1500, Onkyo TX-NR1008, Zone 2 Klipsch AW650.