KT88's in general don't seem to do well in the ARC. I have not tried the Gold Lions but I did try the KT88 Cayin/Shuguangs that came with the Cayin. They were very hard sounding (almost like bad solid state)and they came out immediately.
The Tungsol 6550's were quite good in the Cayin but not quite as detailed in the high end with a softer sound in general and a very lush bass. That said, I could live with that combination but I probably would change the 12ax7's from the the current Mullard re-issues to a crisper sounding tube like a Telefunken (which were too bright when used with the Gold Lions)or RFT. Currently the 12au7's in the Cayin are Telefunkens that I had saved for 20+years!
The Tungsol 6550's were quite good in the Cayin but not quite as detailed in the high end with a softer sound in general and a very lush bass. That said, I could live with that combination but I probably would change the 12ax7's from the the current Mullard re-issues to a crisper sounding tube like a Telefunken (which were too bright when used with the Gold Lions)or RFT. Currently the 12au7's in the Cayin are Telefunkens that I had saved for 20+years!