The specs do not indicate any change in THD between 54x and 52x models. I would not pay too much attention to THD and distortion because that doesn't really dictate how well an amp sounds.
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The AT52x amps are single-ended and use a single board per channel, but you can have up to 8 channels.
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The AT54x amps are balanced differential and use two amp boards per channel, but you are limited to 4 channels total because of this. Each channel will have more power capability (500 watts vs 200 watts) and have more power supply current per channel. Also, since each channel with have two current sources doing push/pull, you will get better control of the speaker. It may sound better than the 52x single ended amps.
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The AT52x amps are single-ended and use a single board per channel, but you can have up to 8 channels.
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The AT54x amps are balanced differential and use two amp boards per channel, but you are limited to 4 channels total because of this. Each channel will have more power capability (500 watts vs 200 watts) and have more power supply current per channel. Also, since each channel with have two current sources doing push/pull, you will get better control of the speaker. It may sound better than the 52x single ended amps.