I tried various tubes, like classic Mullards in the PH-7 and found the stock tubes sound best.
Never tried Mullards, but 6DJ8 Orange Globes or USN white label 7308 Amperex were a substantive enhancement over the stock Sovtek 6922 in my PH7. Nicely extended top end, considerably more detail and improved tonal depth across the frequencies.
Be that as it may, I agree that ARC optimally outfits their Reference components with tubes. The NOS 6H30-DR do offer improvement, but since that tube has really shot up in price the relative value equation may not be their favor. Never tried the NOS Tung Sol 6550 but the GE6550A is not a good choice in the power supply.
Upper echelon Shunyata power cords work very well with ARC Ref - try the Anaconda, Python or King Cobra.