Finally after owning this amp some 15 yrs, added all new Mundorf Gold Silver caps, (The blue Philips caps were not changed as those caps are high fidelity) So my tech guy here in New Orleans, RG decided to test it w KT120's as we were going to list it for sale on Ebay,,,,,due to the shortage of 6550 tubes,,, He had it running w KT120's , I said, why sell , sounds super duper.
Works great.
Here is a short YT vid, gives a hint at performance.
I prefer a quad of KT120's vs 12 6550's.
Let me try to upload the YT vid