The whole Class BD is just bullshit...
Let me put it more simply about the design of the amp, I have been building, selling and sometimes designing audio products, and the whole overload of useless jargon and acronym is just stupid.
Now, from the design and power source of the amp, we can see that this is what we call a BTL arrangement, BTL means Bridge Tied Load. Which means that both speaker terminal have an amp behind it, the black binding post is no longer ground wire, but the reverse signal of the red binding post. It have NOTHING to do with Class B, AB, D, or A... It is just one way to wire up some amps.
The result? It can produce up to 4 times more power than what is otherwise possible with the same power supply voltage.
Why do the designer use this design????
There are several reasons that designer use this setup.
#1: Cost, BTL amp does not need to have both + and - voltage supply, it only need a much cheaper and easily available single voltage power supply. Better still, no complicated inverter to turn a cheap single voltage power to dual voltage, or just complicated and more expensive dual rail power supply. This also means that the company can just pick and choose from the MILLIONS of available power supply in the market, you never have to worry about the power supply sourcing problem. If company A don't have it in stock, if company B want to charge more, if company C refuse your order because it didn't meet their 5 million dollar minimum order requirement, if company D just couldn't be communicated, you still have more companies than you can shake a stick at, that you can get the power supply from.
#2: Cost Per Watt: For the same amount of cost, you can deliver more watts. There are other ways to use single voltage power without using BTL, but ALL of them involves sacrificing some power (Capacitor coupled circuit) or extra cost (Transformer coupled, like in vacuum amps).
#3: Tuning: Because BTL setup will cancel out some characteristics and amplify others, it is sometimes used to tune the sound effect of some amplifiers. And of course, it can also cancel out the desired tonal quality depending on how it is setup. Some amp sound better when in BTL setup, while some very musical ones become as bland as water.