Different processors have different EQs and sound modes that obviously can make an audible difference. But that is no more than saying, if one turns up the treble on a receiver, one can hear the difference between that and leaving it in the normal position.
In the case of modern surround receivers, if one uses the internal EQ, then there may be an audible difference between two models. But in "pure direct" mode, if they are functioning properly and the amplifiers are used within their design limits, then it is unlikely that you will ever hear a difference.
Additionally, since different models in a particular manufacturer's lineup typically use different processing, it would be entirely inappropriate to generalize the sound of the processing based upon the particular brand. Usually, the upper models have more elaborate processing, which, hopefully, will be better than the more simple processing in the cheaper models. But the upshot of this is, if you are comparing one Sony with one Pioneer, you will not be able to generalize this to other Sonys and other Pioneers that use different processing.