FWIW, the GE6550A (now NOS) is not a good tube for the REF 5/SE or REF Phono 2/SE. ARC discourages its use in those applications. I tried the GE6550A in the Ref Phono 2 and compared to stock it was tad warm, soft, and not as spacious. A fine tube in earlier CJ amps such as Premier 11 & 12.
The 6H30 DR works well in the ARC RP2SE and 5SE, but the difference from stock is not, imo, so great to justify today's prices. Hard to believe 10 years ago these were ~$30, but the last of 'em were brought over from Russia some time back by Conus Audio. It is a nice tube with a long life. In the ARC unit I heard it offer a touch less grain and a bit more refinement and extension in the top end.