Subwoofer for TACT digital amp

Would like a subwoofer BUT:
"TacT 2150 puts out an analog signal at their output terminals that is positive at the red terminal while neutral at the black or for the negative halfwave positive at the black and neutral at the red. This is equivalent to the AC voltage of an average amplifier, but:
Both terminals have their neutral at half working voltage, which may appear up to 29 Volts DC. Variation of volume level is mainly done by changing the working voltage. You can expect a voltage between 3 V and 29 V DC on either terminal

Never connect the black output terminal to any equipment ground like any active subwoofer. Even passive subwoofers use one common negative pole if for cost reason a single voice coil bass unit is chosen. Both negative poles for L and R channel are connected to common ground which would short circuit L and R channels of the digital amplifier at the black terminals L to R channel.
If the active subwoofer has ground connected to safety earth, the short circuit will be between both L and R to ground. This case is even more severe because half of Millennium (or 2150) is working against ground then.
This short circuit may lead to destruction (with conventional amps in bridged mode) or the amp will go into protection mode."

What to do?