All the Arcam power amps you mention have the same gain. Of course absolute sound quality varies a bit and has generally improved with time; I see little or no gain (no pun intended!) in trying to use one of the Alpha series in place of the A85 or P85.
FWIW pre owned Arcam power amps go for quite high sums, often not far short of the original retail; so it may be worth looking at the new P80 power amp (2 x 65W rated power) at about £420 or 650 Euros retail as an alternative. It has excellent sound quality plus music sensing and/or a 12V trigger input for power on/off.