Tubes for Antique Sound Lab MG-S115Dt

Has anyone replaced the stock tubes (Sovtek, I believe) on this amp? If so, any suggestions? I listen to mostly jazz and soft rock. I'm looking for a warmer sound, but would also like to retain the reasonably tight bass.
I have this amp. Tried seveal different output tubes, GEC KT-88 far & away the best. Bummer, cuz the cost almost as much as the amp. Tried them on suggestion from distributor.
I replaced the stock tubes with a pair of GE 6550's for the output tubes and Amperex "Fat Boy" (German production) 12ax7's. This got rid of the somewhat strident sound of the original Valve art Chinese KT88's and Russian (JJ)12ax7's (EI 12 ax7's also worked very well with the GE6550's). It also seemed to give more output in the single ended mode. I am still looking for a lower priced "mellow sounding" KT88 as the 6550's I used were 20 year old used outputs from my Audio Research D75. If anyone has any suggestions please let me know.