You'd want to switch tubes to maybe a higher power pentode like a 6146.
There was someone experimenting with a 6 tube 1500 watt amp but I don't
remember what tubes he used just that the filiment current was around 10
amps.
--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794

A higher power tube does seem to make more sense than combining 36 smaller
ones. But unless I'm mistaken, the 6146 is rated for 25 watt plate
dissipation, the same as the 6L6.