There are several different tubes within the 12AX7 family, which is what's in the MPA's. Some are lower gain, some designed for low noise. I don't claim to be a tube guru, I just dove in and tried a few different flavors to see what happened. I think it's good to have a set of low noise and low gain tubes for vocals, even going to a higher quality 12AX7 for smoother high end than with the stock tubes. Is the MPA Gold new or used? If it's used, the tubes could be the noise source. They're usually fairly quiet pieces.
A good source of info on the 12AX7 family is
www.vacuumtubes.com
Just don't ask him what a certain tube sounds like, it really depends on the totality of the tube and the circuit it's plugged into, he can be a grouch on this subject.