Yes, they are interchangeable.
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ARC voices their amps for the 6550.
The 6550 likes to see a lower impedance output transformer, around 3200-3500 ohms. The KT-88 likes to see a 5000 ohm plate-to-plate output transformer.
The KT-88 can seem a little bloated on the bottom and a little veiled in the midrange, compared to the 6550, with the 3200-3500 ohm transformer.
Yes, they are interchangeable but will sound best in different situations.
Check with Leonard at ARC.