I think it's a bit of marketing nonsense TBH. If it's a transistor amp then "DC" may well mean "connected directly to the transistors". Er yes, well they all are. I suppose you could argue that some amps use DC blocking capacitors on the outputs so they could be described as "capacitor coupled", but AFAIAC this is a "so what" feature, there's nothing to suggest either way fundamentally sounds better. Valve amps use higher internal voltages so they need output transformers, so none of them are DC.
