I tend to set the HF level at about 50% and the same with the Volume.  To be honest I use a standard guitar amp most of the time or the DT-50 or now the DT-25, but occasionally I do use the KRK's direct.   I felt that the XLR outs on the POD HD500 were a little bit quiet but with the 1/4" outs I haven't experienced any issues with volume.   No I don't jack up the mixer levels. But it has to be said whan I use these monitors I'm not really intending blowing the tiles off the roof   That said though they do make quite a healthy row if I want them to

I think I found a good solution...I'll get some 1/4 to RCA adapters, they're cheap and then I can use some of the cables I already have.