You can't compare receivers based on the numbers on the volume scale. If both receivers are calibrated to the same reference level then you can compare them for output SPL (should be identical if they are calibrated the same) but the position of the volume dial may not be the same.
Let's assume for arguments sake that both of your receivers have the exact same gain structure and each number on the dial increases or decreases the amplification by exactly one decibel, but one uses a scale of -infinity to 0 and the other -infinity to +16 (as in your example).
When your friends is at -30 he is listening 30 dB below the MAX of zero. With yours at -30 you are listening at 46 dB below your MAX. Remember we said they have the same gain structure and only the scale used for numbering the volume differs. That is a huge difference, but it has nothing to do with the relative power capabilities of each receiver.
If you want yours to be as loud as his at -30 then you need to calibrate yours to the same output SPL at -30. Use an SPL meter to measure the SPL at -30 on his receiver. Then put yours at -30 and adjust the channel trims to read the same output SPL as his. Now they are the same! That is what calibration is meant to do, although you should calibrate to a known reference standard (Dobly Standards - 85 db at 0 on the volume) and he should do the same, except that if 0 is the max on his receiver, he would probably want to try for 85dB at a lower setting, like -20 - you want to leave a little room on the dial to go over reference if you ever want to.
