ARC amps and 6550C Winged "C" small pins

I had the same thing, replaced ARC tubes with tubes from Upscale Audio, no problem. Then replaced the Upscale tubes with Tube store, the pins seemed a little loose in sockets.

In the end, I just left them in (VTM200) and they never gave me any problems.

I bought 8 power tubes from upscale audio, SED 6550, and they feel like they are tight fitting.

Yes. I got into a tiff with the local ARC dealer who gave me a hard time for buying tubes from a known tube supplier. The tech (the owner's brother as it turns out) told me the tubes were the wrong ones for my VT-100 Mk III and wanted to sell me "ARC" tubes for about $700.00 CAD. He showed me what he said were the right tubes and those were the tubes used on the VS series... The next tech I brought the amp to to have the tubes biased showed me that the sockets could be made tighter by closing the metal sheaths a little with a small pointed tool (a skewer works fine). Since he told me he couldn't be bothered doing it himself, I wound up doing it myself: like someone told me on AA, it's not brain surgery.

Honestly, for the money, I think ARC should provide better sockets.

Good luck!