CuBox and ODAC Rev B output too low, Need help


I have just bought a second DAC after the first one performed very good with Volumio and CuBox i4PRO. The first one was an ODAC from JDS Labs in Rev A. Recently they are selling Rev B which has little better technical data.
The problem ist, that the output level (RMS) of Rev B is only 140 mV, Rev A has 2000 mV. Therefore the noise floor is very high and the signal too low for my Harman Kardon HK680 amplifier.
That is only with Volumio. Under Windows and OpenElec the output level is 2000 mV RMS with a 1000 Hz sinus signal @ 0dB, both Rev A and B are looking equal.
And there is another effect: the Rev B must be connected via an USB-Hub (unpowered), otherwise no signal at all is coming out. Under Windows everything is ok. I tried different USB cable lenght, with and without ferrite and different Hubs, powered and unpowered, no change.
So I am a LINUX noob and couldn’t find any parameter to change. The only confifg file I found was the mpd.conf.
After that I wrote a mail to JDS Labs and got the answer:

“…Okay, this must be a power management issue within Volumio.
ODAC RevA enumerates its USB connection with actual power consumption (about 100mA).
ODAC RevB enumerates USB reporting no power consumption. This method provides much wider compatibility with mobile operating systems. It’s important to disable USB power management features in some operating systems, so that the OS will apply power to the DAC. Your USB hub supplies power, so it’s also a good solution. …”

Imho this explains the no signal, but not why the signal is too low.
Is there a parameter which can be adapted to fix the problem, or is the DAC not compatible.