This is completely normal and of no consequence at all. Just turn up the volume. There is nothing you can really do to equalize the volume with the other sources, except perhaps putting attenuators in line with your other sources, which is going to have some other undesired effects. If you want to go this way, google "Rothwell attenuators". But really, I'd just live with it.
Actually turning it up to max might be very good. If your receiver has a conventional volume control pot that is motorized, the higher up it is turned, the more linear it becomes, until at maximum, it is almost completely out of the circuit and maximally linear. Remember a volume pot doesn't add gain, it can only throw away signal.
And another cartridge is not likely to help, as the output of the Rega cartridges is actally at the very high end of output voltage compared to most MM cartridges.
Actually turning it up to max might be very good. If your receiver has a conventional volume control pot that is motorized, the higher up it is turned, the more linear it becomes, until at maximum, it is almost completely out of the circuit and maximally linear. Remember a volume pot doesn't add gain, it can only throw away signal.
And another cartridge is not likely to help, as the output of the Rega cartridges is actally at the very high end of output voltage compared to most MM cartridges.