Spec, linked by
@AdamG, at the beginning of the thread, is 15 Vrms max. output voltage into 8 Ohm. This would be consistent with the measured power, and yes, it probably means a DC boost or charge pump circuit…
The Alto manual just says “Output maximum voltage 15 V RMS”, without saying if it’s for the speaker or HP amp.
I’ll be curious to know more about the internal design: knowing it’s Chord—not exactly known for convention designs—and it uses their “Ultima” technology where the feedback is prior to the final output stage, could it be a case where the HP and Speaker amp’s are one and the same… except for the final output stage?