Adcom made some really sweet little power amps in the 80's, designed by Nelson Pass as a consultant. For instance, the GFA-535 II is true dual mono, 60 W per channel into 8 ohms.
Best bet for finding a standalone headphone amp pre 2000 is finding something from Headroom or you can compromise and purchase a krell ksa5 clone (original is circa 1991 I think) from ebay.
Have you heard of CMoys? The basic ones are okay. Well there have been a few amplifier builders who really took the basic CMoys to another level. In the 1999-early 2K era some pretty powerful CMoys have been built. I had one and it drove a pair of AKG K500 headphones beautifully. Look out for those...
Last one, look out for the Sansui AU-217 or Sansui AU-317 pre-amplifiers. Good for all kinds of systems, good headphone out and a great phono for your turntable. Small and heavy bricks but powerful.
If thinking of using a vintage integrated for headphones, something to be aware of is that the headphone jacks usually have very high output impedance (eg my Rotel - couple of years too young for you - 340 ohms). Not really an issue for your HD250s, but could be with other 'phones.
Congrats mate! I thought of it when I saw your previous post. But I didn't want to say it, as it could've been taken the wrong way. To answer your question, there might be schematics for some DIY stuff. Cavalli and Gilmore come to mind.
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