Answer from ARC:
"The REF150 is a balanced input power amplifier only and requires a true balanced differential input signal to operate correctly. You cannot use XLR to RCA adaptors as these do not generate the required negative phase input signal required for true balanced operation. You have 3 options: 1) Obtain a preamplifier that offers true balanced outputs (an Audio Research Reference model would obviously be the preferred match to your REF150); 2) Use an active SE to Balanced converter that generates a negative phase signal from the positive phase signal by means of a phase inverter circuit. 3) Use a transformer to generate both phases of the signal passively, as you referred to. Our experience is that options 2 and 3 do not preserve all the benefits of option A, with some loss of dynamics and /or transparency to be expected. We do not have any product recommendations for options 2 or 3, so you will need to do your own research to obtain a satisfactory product that performs up to your standards. The REF 150 has a 300K ohm balanced input impedance (150K each leg) and thus presents a very easy load to any device that must drive it."
This again confirmed that I have made a wrong choice. It's a shame I didn't do research before buying the Ref 150. Just pray that the sound will not be too bad with the transformer's option.
"The REF150 is a balanced input power amplifier only and requires a true balanced differential input signal to operate correctly. You cannot use XLR to RCA adaptors as these do not generate the required negative phase input signal required for true balanced operation. You have 3 options: 1) Obtain a preamplifier that offers true balanced outputs (an Audio Research Reference model would obviously be the preferred match to your REF150); 2) Use an active SE to Balanced converter that generates a negative phase signal from the positive phase signal by means of a phase inverter circuit. 3) Use a transformer to generate both phases of the signal passively, as you referred to. Our experience is that options 2 and 3 do not preserve all the benefits of option A, with some loss of dynamics and /or transparency to be expected. We do not have any product recommendations for options 2 or 3, so you will need to do your own research to obtain a satisfactory product that performs up to your standards. The REF 150 has a 300K ohm balanced input impedance (150K each leg) and thus presents a very easy load to any device that must drive it."
This again confirmed that I have made a wrong choice. It's a shame I didn't do research before buying the Ref 150. Just pray that the sound will not be too bad with the transformer's option.