On the cleanbox, I use a Crown PT-2 for my sub. It has a switch inside to adjust what input gain it should expect. Is this Performing the same function. It seems to work OK to me, but I do have the sub out level up some on the receiver.
I certainly don't claim to be an expert but I disagree that a cleanbox or equivelent is required for XLR inputs. I am using an RCA between my Yamaha RX 667 and ED EQ2 (for HPF) then a RCA splitter and 2 DIY RCA to XLR cables from it to my Crown XLS 802. I have the gain at approximately 60% on the 802. I don't have signal strength issues. I've heard of the problem with other receivers but it has not been an issue with either of the Yamahas I have used. Of course YMMV