From what I understand not all AC187/AC188 transistors are vulnerable to tin whiskers depending on the manufacturer and whether or not the cases are sealed with resin. For example the Tesla equivalents on sale here should be ok:
https://www.ebay.co.uk/itm/GC511-GC5...sAAOSwWKpcbIG5
By the way I would recommend replacing them both if one is faulty once you are satisfied that the rest of the audio amplifier circuit is fault free.
Alan