Let me explain . . .
The DS power cut-back, applied to the DSLAM output, is related to the length of the E-side cable from the telephony serving exchange. The sole purpose is to reduce the power output by the DSLAM on G.993.2 (VDSL2) circuits to be equivalent to the power, measured within the PCP on the D-side cable, of ADSL2+ circuits which have originated from the telephony serving exchange equipment. In other words, the measured power at ADLS2+ frequencies in the D-side cable of both ADSL2+ and VDSL2 circuits are equal. Hence the latter does not "drown out" the former.