Use an active pre-amp (SS is better than SET for bass response) with 2 RCA outputs to split the signal from the CD player/Media player/TT two ways - one direct to your 300B and then the Horns, 2nd using Y splitter further to drive each plate amp/subwoofer
My system is now PWD DAC II (has XLR and RCA both with active signal) - XLR to main preamp - 45 tube active and to my AN300 B monoblocks; RCA with splitter ie 2 lefts to left subwoofer bridged amp and 2 rights to right bridged subwoofer amp. Separate volume on PWD DAC, Tube Main Preamp, and adjustable X-over frequency and Volume on Subwoofer amps makes adjustment and blending seamless.
My system is now PWD DAC II (has XLR and RCA both with active signal) - XLR to main preamp - 45 tube active and to my AN300 B monoblocks; RCA with splitter ie 2 lefts to left subwoofer bridged amp and 2 rights to right bridged subwoofer amp. Separate volume on PWD DAC, Tube Main Preamp, and adjustable X-over frequency and Volume on Subwoofer amps makes adjustment and blending seamless.