If you set the load resistance to 47K, there is no real reason you cannot use a SUT with your phono preamplifier, provided, as CLeeds mentions, the phono input will not be overloaded. However, I am not sure from what you’ve written that you really need a SUT, because with 60 db gain, it is not immediately obvious to me that you do not have adequate gain for a 0.4mV cartridge output, on paper at least. (Is the "0.4mV" based on a velocity of 3.54cm/sec, the old standard, or 5cm/sec?) I’ve never heard of the Coph Nia, but perhaps you should consult the manufacturer. Be aware of the effect of the SUT on impedance, if you go down that road. Which is why I say you would want to set the input load to 47K; SUTs like to see that resistance on the secondary side. Likewise, you could also use a "head amp", assuming by that you refer to an outboard active gain stage designed to be inserted between the cartridge and the phono input. But at most, you only need a few db additional gain, if what you have seems insufficient, and if the cause can be determined to be lack of phono gain.
Check to see if there is an impedance matching problem between the output of the Coph Nia and the passive attenuator or between the passive attenuator and your amplifier. Also, check the input sensitivity of your amplifier. (These are other causes of that "lack of gain" feeling.)