Leben CS 600 tube rolling

I am thinking of trying a new set of power tubes in my Leben cs600.  As the NOS EL 34 and 6L6 are drying up ( or getting too expensive ) I thought I would try new production KT 66 ot KT 88.  In part just for fun and to check out a different sound.

I have the output tube compatibility settings chart for Leben.

For the KT 66 and 88, under the plate voltage column, they have a delta sign.  I am not sure what that means?

Has anyone tried these alternate tubes?

Thanks

MP