Could you compare them to the Lotoo S1?

Sorry, don't know the Lotoo S1.

Something wrong must be with your nano black. Nano black is more powerful vs hip dac on SE 3.5 i even listen it right now tried with all HD6 series and Sundara . I even tried the older firmware to see if thats the reason and again Nano was stronger .
Is your hip dac pre production review unit or something special ?

Check the specs of both. I don't know when the hip-tac I had borrowed was produced.

thanks for replying... i go to Ifi web site headphone app and it shows that nano is more powerful unbalanced vs hip dac unbalanced ... So ok thanks.

ifi audio are not very clear about it. I asked them via FB messenger and will let you know the answer. I would not worry too much about power, both drive the 300 ohm Sennheiser HD 600 well.