Thanks! - the conclusion seems to be that 6550 is not less musical :-)
Maybe the idea (Jtimothya) to try tube rolling the 5751 and 6FQ7 is a better way...
For the time beeing I have JAN GE 5751 (new production I guess) and RCA 6FQ7 (Brasil, red color letters)
Comments on these driver tubes?
Ole