I don’t understand why you decided to cover the usb output from the streamer to optical, when the Dual Core has a usb input. I would get rid of it, and just connect via usb.
USB is a very good digital connection, and measurably better than optical (and don't waste money on the scam of fancy digital interconnects).
Did you ask DSpeaker in Finland? You can just give them a call and you get to talk to one of the engineers.
Personally I am not a fan of tube amplifiers, but the biggest sound quality improvement usually comes from better speakers. In your case, that is where I would go first, apart from selling that usb to optical converter.
Personally I am not a fan of tube amplifiers, but the biggest sound quality improvement usually comes from better speakers. In your case, that is where I would go first, apart from selling that usb to optical converter.

