"It seems to me that your phono board is not powered up as it should be.
The really low volume is like you feeding your phono signal directly into a high level input, without the amplification of a phono preamp circuit."
Thats not the problem. A phono preamp doesn't just amplify the signal to bring it up to a line level source. It also runs the signal through an EQ curve. If there's music playing and the only issue is low volume, then the phono pre is working. If you have low volume and the music sounds really messed up, you could make a case for a phono pre issue.
More likely than not, the phono stage in the receiver is just not that good. If you were to put a decent, entry level stand alone phono pre in the system, that should take care of the problem. It doesn't have to be anything really expensive. You can get something workable for about $100-200.