I think that bastion of high-priced replacement semis TCG outfit had a few Sanken subs---but you are right, they are hard to get and not multiple-sourced.
I have a stash of Sankens somewhere that at least I know are complements, and are probably close to what you need, if all else fails. Doesn't look like I'm going to use them soon---I've had them for years since UREI was throwing some stuff out.
EDIT: I located them. They are the 2SA1216 and 2SC2922. They are 200W, at least 17A and 180V devices. 40MHz F sub t! You are welcome to a few of them.
EDIT II: Oh wait. I was thinking these were your big devices you needed the insulator for. Never mind---these are definitely not subs for the 1668.
2sc2238/2sa968 are (old) drivers usually used for driving the 2sa1216/2sc2922 output pair. They are good substitution for xxx/2sa1668 also. Newer drivers are the 2sc3298 and 2sa1306.