I purchased one of these multi channel Amps second hand. It's a Rotel RMB-1066, I wouldn't call it HiFi more like MidFi, but its a cheap way into Tri-Amping or Active crossovers. This model was mainly sold for home cinema use, where folks were wanting to get into separates and split the amplification from the receiver/control section. It has several features that make it attractive for multi channel music use though.

First it is set up as 3 pairs of channels, so it's like having 3 Stereo Power Amps in one box

Second, each pair has it's own gain control, the pots are adjusted using a screwdriver and are located between the front fins, as indicated on the picture. For three way speakers this allows adjustment of each driver pair from one pot, so left and right bass drivers can be adjusted together as an example. Once drivers are initially matched for sensitivity the volume is controlled from your pre-amp.

Third is the RCA inputs, one pair of inputs for each pair of channels, again more like a stereo amp x3 which is good for going Active.

Lastly the input pairs can be linked by sliding a couple of switches so that one pair of inputs serves all three amp sections, saving on cable runs etc and useful for Tri-Amping from a single pair of outputs on a Pre-Amp.

This is a very versatile piece of kit, you can also bridge any pair of channels to increase the power in that section but reducing it to single channel. So you can have 6,5,4,or 3 channel output depending on which pairs you bridge. Power output is 60W/8 Ohm x6 or 150w/8 Ohm x3 (Bridged).