You might ask EAD if it's ok to short the balanced input. My guess is that no harm will be done since it's the input not output. Most inputs are designed to be shorted. Krell recommends shorting balanced inputs when using RCA.
Shorting Balanced Amp Imputs (pin1-3)
I have an EAD Powermaster 2000 that has both rca and balanced connectors. I have a speaker buzz-hizz that is present when rca connections on the amp are used (even with powercord ground lifted) but virtually dissappear with balanced connections. Will shorting the balanced imputs on the amp (when rca are used--my pre-am does not have all 5 channel xlr output) either (A) damage the amp and/or (B) possibly eliminate the electronic buzz-hiss noise I hear. The amp has been back to EAD tested, and over $1500 or repairs made.