Lacking in resolution. Other valve amps I have had (and still own) do a better job in detail retrieval making the 509/II sound a bit vague and amorphous. It isn't the ultimate detail machine which means that the soundstage too is vague and ill-defined (relatively speaking). The overall presentation significantly fails to let through some of the excitement and pizzazz that is in the music.

I don't think the 509/II has a unique sound at all - ime it's a rather old fashioned valve amp sound. Perhaps I would call it the 'missing link' between the old fashioned soggy-bass, moderate rez mids and smoothed off treble of all too many early valve jobbies and the new generation of higher rez valve amps that can go face to face with the best solid state and beat them on 'musicality'. "A bit on the slow side, but sturdy" might be a decent summary of my view on the 509/II.
LOL. I don't know what was going on there, Jerry, but the above is *so* diametrically opposed to the sound of (original 1978) 509s in my mate's system (with DBLs and a Croft pre) that it beggars belief

Quite frankly, I don't know what to say... Which is somewhat unusual for me!

I agree wholeheartedly with you though about:

the new generation of higher rez valve amps that can go face to face with the best solid state and beat them on 'musicality'
That's exactly what my Yaqin does. However, in my experience 509s don't sound anything like you describe. In my mate's system they are a bit 'dry' and clinical due to their studio pedigree, but are also very lively, detailed, neutral and punchy - a million miles away from the world of "soggy bass" and "smoothed off treble" you have alluded to...

Marco.