@scrumpymanjack has this Quad preamp of yours been refurbished since new? If not, its in dire need regardless of the condition of the tubes.
I am repeating this; I forgot to mention that if the unit has not been properly refurbished recently, the filter capacitors in the power supply could cause the power transformer to fail. I would not run the preamp until this work has been done; a power transformer for this preamp will be a difficult thing to find and likely expensive when it finally turns up.