Information is king, so I would like to ask the OP some questions before suggesting anything else:
1) Is pure direct mode making any difference, in terms of how loud?
2) Since you are using ARC, have you check the setting of the TV's sound menu, also, have you tried setting the TV volume to "Maximum".
3) Is the unit purchased brand new and came in a sealed box, or a demo?
If 1) and 2) are not making a difference, that is, louder, or loud enough even with volume set to above 80 then it is time to do a factory reset, because some settings might have been messed up somewhere and it is hard to troubleshoot on a forum so it is easier to start from scratch, that is, return the SR8015 to factory condition.
No
Since you have tried two Marantz AVR already and they behaved the same, the chance of having two units being defective should be low, though cannot be completely ruled out, but let's do the obvious thing, such as factory reset first and go from there.
The Onkyo RZ3100 may be slightly more powerful but the difference would likely be less than 1 dB, hardly noticeable, all else being equal. So that is definitely not the reason.