Not sure about the 6LYA, but my 4LHA-STPs vibrate quite a bit at idle, then smooth out tremendously at about 1,000 RPM. They run silky smooth above idle, they just shake at idle. I recently had the valves adjusted and that helped a lot, but my mechanic describes them as "shakers."
That said, in the 8 years that I have owned them they have been the best marine engines I've ever run, inboards, outboards, gas, diesel, whatever. I love them. The same mechanic says that they are among the most trouble free engines he works on, including all the Cats, Cummins etc. He thinks it is one of the two or three best engines Yanmar has ever built.
I'm not in any way knocking the 6LYA, just passing on my experiences.