FWIW - I replaced the original 6550's in my Sceptre with Tesla KT-88's. After fooling around with the bias a bit, they are set fairly high. The sould is very good.
I defer to the techs, but if your Solarus was the big amp, factory set for 6550's, they you should be able to run these tubes. For guitar, at least, they sound better (to me) than the Svetlana 6550's.
As always, YMMV.