maybe you misunderstand what they mean...
But it does mention something important that after 2000 the power supplies of Onkyo receivers had been changed and the volts going into all channels are reduced which means much less watts. That's a really bad news for the users. Before year 2000, Onkyo receivers had power supplies to output the rated watts to run all channels.

By the way, the specification of Onkyo TX-SR600 is as follows:

•145W／每聲道（前置），145W（中置），145W／每聲道（環繞聲道），145W（後環繞聲道），6Ω，JEITA•總諧波失真：0.08％（所有聲道，額定功率時）•所有聲道均為全帶寬•寬頻放大技術（WRAT）•頻率響應：10Hz–100kHz（+1，–3dB）•無測量設置•大電流、低阻抗（6Ω）驅動能力•所有聲道的放大電路均採用分立元件組成。優化增益音量控制電路•有隔離的重型電源變壓器•優質的衝壓鋁制散熱器•低頻效果聲道（LFE）電平控制•高低音調控制。分頻點調節（80／100／120Hz）