Jerry, this is more a problem with construction, than design. In theory, a power amp can have the input noise of a very high quality phono stage, which when REFERRED to its rated power output, can give a HUGE S/N. However, it is not really that important, unless you are using HORN loudspeakers, which amplify noise of the power amp, because they may be as much as 20dB more efficient than planar drivers or most direct radiators.
The KEY is the S/N of the preamp, because it has an extra 10-20 dB of gain in front of the power amp. This is where things fall apart, because of the VOLUME CONTROL which adds up to 1/4 the effective loading resistance of the volume control, 50K/4 is typical, that usually limits the S/N.