Hi - check out the Sterophile review of the BAT 3iX. Although this pre is different, the balanced output has 6db higher gain when used with single ended input.
Furthermore, the output impedance of the preamp varies significantly with balanced/unbalanced across different frequencies and in the testing there was a significant hf roll off in balanced mode with low impedance input power amps. Your power amp is 31k input impedance and the VK3iX is high and goes to 14k at some frequencies.
If the 3i is similar to the 3iX then you probably have a hf rolloff issue in balanced mode with the Musical Fidelity amp.
Furthermore, the output impedance of the preamp varies significantly with balanced/unbalanced across different frequencies and in the testing there was a significant hf roll off in balanced mode with low impedance input power amps. Your power amp is 31k input impedance and the VK3iX is high and goes to 14k at some frequencies.
If the 3i is similar to the 3iX then you probably have a hf rolloff issue in balanced mode with the Musical Fidelity amp.