Disadvantages of buying old solid state.

I am looking to upgrade from a single ended amp to a balanced because of the purchase of a new balanced pre. Due to budgetary reasons I am forced to buy a used unit. I am interested in the Pass .5 series as what I've heard of the .8 I was not happy with. There are those who prefer the .5. sound as well. I was told however that buying such an old unit I might find that the bias has changed. Any experiences with buying old solid state and any negative consequences? I would very much like to hear anyone's views on the matter.

Thank you.