Problem with phono stage

Doesn’t the black cube have multiple gain settings? The overload margin on it for a MM cart is much higher than 5mv.. something like 45mv. I suspect you have the gain setting too high on the cube...set it to 36 or 46db and see how that works..

Hope this helps.

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.

Just for data purposes around the issue of capacitance in this particular issue -

I asked AudioQuest support some time ago about capacitance in their cables, and was told they have 40pf per meter of capacitance. Not sure I believe it is that low, but if true it is quite good. I use AudioQuest Big Sur for my phono cables and enjoy them very much.

I believe the Arcam A28 has 100pf of capacitance in its phono stage (its not published that I could find, but looking at the schematic that is what I see) - so with an AudioQuest cable of 1 meter, that would present 140pf capacitance to the Nagaoka.

The Black Cube Statement claims the same 100pf of capacitance so with the Black Cube in MM mode the Nagaoka would see the same amount of capacitance.

Given this and the fact that with the built in stage sounds very good and is very quiet with the same cables, I am not sure the issue in this case is cables; and given the Cube's reputation, I suspect something is not right with this specific cube; 

I haven't posted much here, but do read quite a bit and love to learn; I have learned a few things on this thread and find much of what @atmasphere, @geof3 (and others as well) have posted to be helpful and make sense to me. However, @rauliruegas, you are obviously knowledgeable, and have a lot to offer, but I would prefer you leave the personal attacks out of your posts. In my opinion, personal attacks don't help you, the OP, me or others and cast a shadow over the rest of your posts. 

I am still scratching my head a bit as to why when the only thing changed between the two scenarios is the phono stage that we get an "unlistenable" experience; it could be that the Lehmann requires a different setup I suppose, or that it does a much better job at exposing other weaknesses in the system - but I would find a way to verify the Lehmann is performing to specification before proceeding down other avenues. I have bought (and fixed) a lot of used equipment, and found that sometimes what sounds fine to the seller can mean they did not know or could not hear that there was a problem.

The A28 phono stage appears to be a simple single gain stage op-amp based design, but owning and listening to a few reputed 'better' ones (A Sutherland PH3D and a Croft Micro 25 for example), I think it does quite well. I would not say the built in stage is better than the Lehmann, but if that is so, I would think the Lehmann should sound better through the A28. My other external stages certainly do. 

Excellent - glad you were able to work it out! Kudos to you and Geof!