FWIW, I was interested in this unit at one time and read a lot of user reports as well as reviews - it seems that your experience may not have been all that unusual. I noted a lot of comments about tube selection and the need for NOS (relatively expensive tubes) to get low noise and better tone than new production tubes. You might consider expanding your research and asking owners for specific tube recommendations. I recall some of the recomendations for for NOS RCA clear tops. BTW, I'm using some EH's in my Cary CAD 50II amp (with 6550 power tubes) and do not feel that they are overly bright.
Also (and I'm sure you are aware) you might compensate this brightness band by something as simple as speaker toe in, i.e. moving them so that your hearing thim a bit more off axis.
Also (and I'm sure you are aware) you might compensate this brightness band by something as simple as speaker toe in, i.e. moving them so that your hearing thim a bit more off axis.