FWIW, I set the gain so that the volume control's setting is about 12 0'clock. The actual gain setting involved needed depends on the output of the cartridge (or other sources), the input sensitivity of your amp and the efficiency of your speakers.
But, FWIW consider I find the Belles amps to be less than tonally optimum even though electronically they will handle the high out-put impedence of the Magus. They certainly aren't going to add warmth. But then I don't care for most SS amps either - they are all too 'dry' for me, even when they are not plagued with grain or mid/high end emphasis.
Don't overlook tube selection in the Magus. I don't know what you are running, but tube selection in both the phono section (especially) and the line stage can make an important difference. You might try a couple of new production tubes in the line stage first, like the JJ6922 which has a warmer signature than many, and see if that makes a difference. If that floats your boat, you can then seek to refine your selection in NOS tubes if you want more.
Hope that helps a bit.
Have fun - its a nice little pre-amp.