I wouldn't worry about the difference in line-level standards. "+4 db" undoubtedly refers to +4 dbu, which is a line-level standard for professional equipment, and which corresponds to about 1.23 volts. A consumer-oriented cd player or other digital source, probably including your USB DAC, will commonly output a full scale (maximum) value of 2 volts or so, which is considerably more than +4 dbu!
"-10 db" undoubtedly refers to -10 dbV, or 10 db below 1 volt, which in turn is approximately 0.32 volts. 1.23/0.32 corresponds to a difference of a little under 12 db, which simply means that you will have to reduce your level controls to settings that are 12 db lower than you would have to if the you were putting a full scale voltage of -10 dbV into the Carver.
But keep in mind also that you will not necessarily be putting a +4 dbu signal into the Carver. The full scale (maximum) signal that you would put into the Carver will correspond to the full scale output of the DAC reduced by the attenuation provided by the passive attenuator, and further reduced by the attenuation corresponding to the level control settings on the CX310, the adjustment of which will in turn take into account the desired volume, the sensitivity of the speakers, and the gain and sensitivity of the Carver.
In general a more significant consideration would be how to interconnect the balanced and unbalanced interfaces.
This paper may be of interest. Fortunately, however, I note that in the case of your CX310, according to
the manual, all you have to do is to utilize unbalanced cables terminated with 1/4" TS (tip-shield) phone plugs, and you're all set!
Regards,
-- Al
