censtadOn the Pioneer website for their Elite receivers, they have a compare function. The 701, 801, and 901 are essentially the same except for power output and the 901 is 11.2 audio channels, while the 801 and 701 are 9.2. The 301 and 501 are 7.2, and they have fewer audio/video features than the others. The 501 can receive firmware updates to process HDR, I believe, but the 301 is not in their Elite lineup anymore. Beyond that, you can go to the Pioneer site and do a comparison.
censtadI'll try to do this just like I would a 5 year old.
Go to Google, type pioneer receiver, then on the webpage, click compare on the ones you want to compare.
If there is something specific you have a question on, that isn't there, come back to us