Scott, at least two things about your question make it difficult to give an informed response. The first is that you just "feel" that it should be louder, but you apparently don't have an SPL meter to actually measure if the volume is where it should be for normal listening. A similar problem exists in another thread involving distortion, which also has no information about the actual sound level to help with analysing the problem. Another point is that you don't name the music CDs involved so that if any member also has one he can give his experience with it.
Music CDs have no established reference level such as movies do, but as some replies indicate, many pop recordings are mastered at the maximum level(or even a bit above it, into distortion from digital clipping), so if anything they exhibit an appalling uniform excessive loudness level.
Onkyos have several settings(muting, volume limiter, IntelliVolume)which affect volume, but these all have to be set differently by the user for any unusual limitation to exist.
Most home movie listeners find the 0 reference level entirely too loud for standard usage and set it quite a bit lower than even the -5 that you mention. Sure you can set it higher than that for the music. The 807 almost certainly has plenty of power to handle it and neither it nor the speakers will be damaged, but if you're already playing at a loud level, your hearing may be.
