Ayon Spirit-1 tube rolling

I just learned that the Ayon Spirit-1 will accept only KT88 tubes and not 6550 tubes, the reason stated was that the voltages on the 6550 are different. Encountering an amp that would run KT88 and not 6550 was a surprise to me and I thought I would use this as a learning opportunity. So, could I ask those members with greater knowledge to please shed some light on this?

Thanks for your help.