Good quality KT88 power tubes?

MArcorin is right. Kevin at upscaleaudio.com is THE tube guy. Hi quality rare tubes. I am using the EI KT90's and they are superb. Smooth, detailed, and great punch. Highly recommended. There's also Genelex but you'll pay quite a bit for em.

The best KT88's I have used over the years are Genelex Gold Lion. They are to KT88 what Western Electric is to 300B. For smoothness and musicality while retaining definition and punch I have heard NOTHING to compare.

I will chime in with another reccomendation for Kevin at Upscale audio and his choice of KT90s.

Had the Sovteks in a pair of Quicksilvers. Indeed they are not good sounding tubes. Bought a set of Aeros and they are quite an improvement. Reasonable price was a factor also. I am not disappointed in them although I have heard good things about the KT-90 also.

I suggest you check with the designer of your amp, and see if you can substitute other tubes in place of the KT88. Possible alternatives (beyond the KT90 already suggested) are, 6550, 5881, and 6L6. Old stock American tubes (used, good) are superb, such as the Tung Sol 6550 and 5881. Many designs that use KT88 can use these tubes. Old stock American tubes can often be found at Antique Electronics in Arizona. The comments about the Genelex Gold Lion are absolutely correct, but they will be (probably) over $150.00 each tube, maybe much more. Good used US Military 5881 show in Antiques catalog at only $17.00. If you must only use KT88, listen to the Tesla KT88, or even the Svetlana 6550, I think that both of these are better than the Sovtec.