How can an amp that measures flat throughout the frequency range get voiced?
Point taken. I would seriously doubt that consumer fi companies are taking the time to voice their offerings. That being said I would believe in this day and age of EQ, DSP, whether there is an toroidal transformer, use of a class a, class d, class ab etc... would lend a character to an components sound, so yes to my ear different brands do have unique perhaps unintended sonic characteristics. Measurement of flat signal before clippings as real as that is, does not exactly apply here. It is sort of like arguing whether a brass trumpet sounds different from one made of silver. Then trying to measure the result, which will show there is no difference in the sound wave. Some musicians swear they can tell the difference and have been able to reliably identify which is which, while others cannot. I am of the camp, that appreciates scientific measurements can provide proof of a sound design concept, but what I believe in the most, is what I can hear.
Point being. The Anthem receiver, I currently own for home theatre sounds different from the old Marantz it replaced, and yet again different from the old school 2 channel Magnum MD208 in my wife's system, and yet again different from the vintage Pioneer integrated in my home office. Why? I would agree voicing may not be the right term here but there are differences in how these four components sound.
Four factors I think would lend an character (Voicing )to an receiver. Whatever is happening to the signal before it hits the amplification stage. The quality of the amps used and their power supplies, and isolation from grunge coming from within it. It could be said type of amplification used could be considered too. I am sure some would argue an class A amp stage sounds better/different than AB, than D, yada, yada, yada...
Even in the High end world of minimalist stereo receivers. The Magnum Dynalab my bride uses sounds a bit different than the new McIntosh receiver I been drooling over at the HiFi salon. I suspect it is due to the differences in the parts, materials and technologies used in preamp and amplifications stages. One is using parts from 20 years ago and the other is using ones that are in current production.