I heard people are using them instead of 6550
Yes, but I also heard that it should not be done from knowledgeable people. In fact there is a reason for it : the 6550 has a maximum grid G1 rating of -300V and the KT88 has a maximum grid G1 of -200V . Under some specific conditions at power up and down the grid G1 of the power supply can be at -250V .
It is why ARC had to modify some of their amplifiers when moving from 6550s to KT120. In the power amplifiers using KT120 or KT150 they still use the 6550 in the power supply.