I really did not set out bridge vs higher end amp. I really want to have a technical discussion on the merit of bridging. Before this thread, I just know Nelson Pass use bridge on his highest end models. I did not realize it actually increase the power by 4 times, but much harder when driving 8 ohm speaker. But as the thread progress and I really was thinking about this for the last few days that lead me to draw the conclusion from last night.
Below is my thinking only, I can only speculate. Feel free to add your idea:
At some point THD is not nothing. If THD is below 0.005% you can argue it's nothing. But if THD is like 0.1%, it is really not nothing. Just by a single frequency itself like we test with FFT, it sounds like it's very low, it's like the signal is 1V, the harmonic is 1mV. That really sounds like nothing. BUT music is a complicated spectrum with many frequency peaks. If you take many many nothing mix together, it becomes a constant hum or hiss like background varying with the input signal. That will become an issue. It's not just the harmonics of the signal, it is the IM( inter modulation) of the harmonics that actually create new frequencies when the harmonics mixing with each other. It will muddy up the music. In particular if the new harmonics is close to the main signal frequencies. It can changes the sound.
I am not an expert, I am still learning about audio. But I suspect this is what's happening. Like old tv vs HDTV. HDTV has a lot more bandwidth, you would expect the edge is a lot sharper. But in real life watching, it's a lot more than sparper, it actually looks more 3D, everything just looks deeper. Being more pixels not just make the image sharper, it greatly reduce the smear in the image like the low BW of the old tv standard. Back to audio, the harmonic mixed together form a background that vary with the original signal and form a smear that reduce transparency and sound stage.
One thing I notice is the better system feels like it's 3D, that if you close your eyes, you feel like you are in the concert and can hear different instrument in different spots. It just sounds a lot deeper.
Another thing that the books of Doug Self and Bob Cordell talked a lot is crossover distortion. It is the most offensive distortion for hearing because it creates higher harmonics like 7th, 9th and beyond. Bridging does not help in crossover distortion, so far, running class A or optimal biasing class AB ( Self called class B) minimize crossover distortion. Optimal biasing dissipates quite a bit of power...bigger heat sink, more output pairs.
Again, this is my speculation only.