Question from a 220 Volt country

Connect the amp's speaker outputs to the sub's high-level input? Should be no problem. The voltage in the amp's speaker outputs is the same whether the amp's power supply is 220V or 110V. On top of that, you are feeding your amp 110V anyway, via the stepdown transformer.

But watch out for those Internet audio forums--you never know who's advising you. ;o)

That's true... you never know! ;-)

Yes, the Neutrik Speakon cable connects the speaker outputs of the power amp to the high level input of the sub. But the amplifier operates in 220 Volts (no tranformer there) and the sub, in 110 Volts, fed through a stepdown transformer.
Apart from that, I guess you are right. Thanks for your help!

Woops, sorry, got the input voltage requirements reversed. Thanks for pointing out that the sub takes 110V and wears a stepdown, not the other way around.

And also, of course, that still doesn't change the voltage in the connection you are proposing to make from the amp to the sub's Speakon connector. It should work fine.