"Even unbalanced signal on the conventional RCA input is converted to the differential processing mode right after the input terminals. (Emphasis added.)"
That would be a neat trick indeed, without a transformer to do the job. I suppose it could be done with an active device, too, but that would be suboptimal, because it requires a processing step in the signal path which would inevitably add distortion however minor. Are you sure the reviewer did not just misunderstand some blurb? The fact that the PE505 is fully balanced does not at all mean that the RCA input jack is not there to receive an unbalanced signal. Like I said, Atma-sphere and probably BAT, both of which companies make only fully balanced preamps and amps, provide that SE input option. The PE505 further offers SE outputs. This is done so as not to alienate potential customers who do not want to convert their whole system to balanced op. Anyway, this is important only to potential buyers of the PE505, of which I am not one. I'm out.