I tried using one of the new ones with an old complimentary [sic]
one from the good side and this caused a massive imbalance
in the idle bias 1.4v for the good one and 0.6 for the suspect one.
Can I assume from these voltages that the replacements are not
Darlingtons? 0.6v is right for a single transistor and 1.2v
(approximately double) for a Darlington?

It sounds as if these aren't Darlingtons.

Do you have a way to measure the current gain? It should be at least 1000 --
probably higher.