“Given that the loudspeaker cables are connected to the LFs and the Jumpers to the HFs, ”
Before you invest into new set of speaker wires, what is your speaker manufacturer recommendation to connect speakers with a single wire? For example, my speaker manufacturer recommends,
“For optimum performance in single wire mode, loudspeaker cable connections from the amplifier should be made to the high frequency (HF) terminals of the loudspeaker.”
If there is no specific recommendations from your manufacturer, I would try and reverse your current configuration. Connect your speaker wires from amplifier to HF terminals and your jumpers from HF to LF terminals. Report back, if you hear any improvements.