Question for ARC VT 100 Mk II owners

Are any of you using KT120 output tubes in your VT100 ?
Please share your experiences. How do they sound? How about tube life?
I replaced my Svetlana 6550 winged C with Tung Sol KT 120.
I got 800 hours on the tubes and everything is working perfectly.
I love my amp but always felt the low end was a bit week. Not so much anymore. When my friend heard my amp with the KT 120 he said " shit, solid state lows". It sounds more robust and powerfull.
Calvin at ARC told me the KT-120's should not be used in an VT-100's. I'm sticking with my 6550's till further notice.