In some applications, the speaker outputs from the power amplifier are used as source signals for devices such as REL sub woofer. For such application, usually the receiving devices should have DC offset blocking capability because some amplifier by design has floating ground (DC offset) on the speaker outputs. 

The following models have no DC offset on the amplifier speaker outputs and the negative terminal is also the ground:

NuPrime STA-9, MCH-K38, IDA-16

The following models' speaker outputs can not be connected directly to sub woofer input:

NuPrime Reference 20, ST-10, IDA-8