Edited following some further investigation, I’ve solved it!
Beomaster 2000, the sound was poor (distant, phased) when listening thought headphones, but fine when listening through loudspeakers. It turns out the newer headphones (B&O Form 2i) made a poor connection due to the 4 pole 3.5mm plug intended for smartphone connection. I bit of fidgeting with the adapter solved it – sounds fine now.
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