Normally the primary of the output transformer is connected to B+ voltage and the plate of the output tube. In a single ended amplifier there is direct current passing though the output transformer. There has to be a gap in the core to keep the core from being saturated. Then you need an expensive large transformer to pass low frequencies.
Parallel feed (parafeed) has the DC passing through a separate inductor from B+ voltage to plate of the output tube. There is a DC blocking capacitor to the output transformer. Since there is no DC passing through the output transformer no gap is required in the transformer core so the transformer can be smaller and cheaper.
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