The previous posts are correct...it likely needs new tubes.
That being said, I will also say that I owned an SP-6B
a number of years ago, and despite it's exalted reputation,
I found it lacking, especially in the relatively high
noise floor. I later owned an SP-14 which I found superior
in most regards. In it's day (late 70's) the SP-6 series
was quite good, but more recent designs substantially
better the SP-6's.
That being said, I will also say that I owned an SP-6B
a number of years ago, and despite it's exalted reputation,
I found it lacking, especially in the relatively high
noise floor. I later owned an SP-14 which I found superior
in most regards. In it's day (late 70's) the SP-6 series
was quite good, but more recent designs substantially
better the SP-6's.