Hmm - I looked up the manual for the Black Cube and got the gain settings from there - perhaps I saw an older or newer version. This manual talks about not only the dip switches, but also a gain jumper that adds 10db.
In any event, I also have an FMJ A28, and while its phono stage is quite good, and very quiet, my other external phono stages into the aux inputs (A Sutherland PD3D and a Music Hall PA2.2) either sound better (PH3D) or roughly equal the internal phone stage. You should get great sound with the Black Cube and this amp. I am using a Shure V15III and a Grado Gold 3 on my tables at the moment.
The A28 has a numeric volume read out, which the manual claims is in decibels when in the reference mode. I believe the internal stage is roughly 40+db of gain - so I would think the Black Cube into the Aux input and the internal phono stage should be equal in terms of loudness at the same volume setting on the A28. If they are roughly equal, and you still hear a lot more noise with the cube, then I don't think the problem is cables or the cartridge or the amp.