First does your power supply provide enough -V's for the LM1875?
Most SMPS's only give about .5 to 1 amp not nearly enough current.
To get rid of the SM hash you need to know the power supply's switching frequency and build an LC circuit to roll it off.
PS I do believe NUUK uses two SM in series to get the proper + and - current and voltage. If your using one supply your chip is probably rolling out of balance witch will create great amounts of noise and problems.
I did get away with using a single Skynet 8080 to power each channel of my system for many months. ie I was using +/-12volts and that negative regulator held up. BUT I was using very efficient speakers and never needed to turn the volume up very high.
Now, I would rather use multiple units in series to avoid using those rather limited negative regulators!