"I don't think the Rega Mini Fono has the ability to lower the gain on the analog connections. I think the gain control only works with the usb port."
You can sometimes control each input on a HT preamp individually. I keep coming back to this gain issue because 5.5 MV seems pretty high. I haven't uses a MM cart in years, so I guess that could be normal for them. The highest output cart I can remember having is a DV 10x5 at 2.5 MV, and you needed to set the phono pre on MM to use it.
Maybe try fooling with the xover on the sub. See if you can set it to have a limit on how low it will go. It may be having a difficult time with really low notes, and that distortion is somehow effecting the system. Also, it wouldn't be a bad idea to try connecting the sub normally, without using the lfe, and any other type of EQ/Room Control processing.
You can sometimes control each input on a HT preamp individually. I keep coming back to this gain issue because 5.5 MV seems pretty high. I haven't uses a MM cart in years, so I guess that could be normal for them. The highest output cart I can remember having is a DV 10x5 at 2.5 MV, and you needed to set the phono pre on MM to use it.
Maybe try fooling with the xover on the sub. See if you can set it to have a limit on how low it will go. It may be having a difficult time with really low notes, and that distortion is somehow effecting the system. Also, it wouldn't be a bad idea to try connecting the sub normally, without using the lfe, and any other type of EQ/Room Control processing.