I tried a pair of Mk III 's with my Quad ESL 57 's, but preferred the sound of a stock Dynaco ST-70. It's been many years now, so I couldn't tell you the differences. Only that I owned both, and liked the ST-70 better with the Quads.
Edit: Note that the stock ST-70 and slightly modded MK-III 's were what I had back then. I took it to mean that I preferred the EL-34 sound over the 6550 with the Quads. The speaker's strong point is their amazing midrange (I've never heard better). One of my ST-70's has been converted to the VTA input stage, and it would be very interesting to conduct that experiment again, once I add the VTA boards to my MK-III's. Alas, my Quads were crushed by a moving company who failed to crate them as I contracted for.
Last edited by 1973shovel on Sun Jan 08, 2012 9:43 am; edited 1 time in total (Reason for editing : Addendum)