KT88 vs 6550 vs ? for warm midrange - using Cronos Mag lll

I've been listening to an ST70 with EL34 Tubes & really enjoy the Sound .

Using the Cronos with KT120's  it's a different sound / feel - sounds wonderful but seems either more balance / neutral overall . Maybe more lower end but not as sweet in mid range as the 34's 

34's in the cronos is possible but not really advisable [ too much current into tubes - 1/2 the life span  even with proper biasing ] 

I've been running the amp in Triode mode and seem to prefer sound .  Listening levels are moderate / low  avg 75 db 

I'm good with the kt120's but maybe another tube would bring more mids to the game 

* Note - I have changed All the preamp tube to NOS - It has been a significant improvement !-  Telefunken is at center  12au7 , flanked by RCA clear tops , 2 GE ax7 's 

Any suggestions for power tubes is most welcome - Rogue audio is shipping  kt88's in lieu of the 120's [ for yrs due to war etc.] 

what's gained or lost when changing from the 120's ?

forgot - Primary Speakers - Dynaudio special 40's , primary source is Vinyl 

Thanks