In Birmingham, as a teenager in the late sixties, I too recollect it as 'heavy rock' which in effect meant 'serious rock' as opposed to 'popular rock'. 'Heavy' as opposed to 'light' in seriousness.
And 'heavy' was not limited to rock. You could be a singer-songwriter and be 'heavy', if you were serious enough e.g. Dylan.
So to call something 'heavy' was to call it serious, with the implication that it was 'better'. There was no mention of metal.
There was a Birmingham record shop called Heavyhead; a 'head' being someone who was into 'heavy' i.e. serious music. Owned if I recall correctly by Bev Bevan.
'Man, that's really serious man'.
Your mileage may vary.