Sometimes less is more...
Decent jumpers allow both speaker terminals to look like a single electrical connection (almost) while with a bi-wire you are at the mercy of the "voicing" introduced by the additional wire. I am guessing in your case the speaker wire used is not the best match which became obvious in the bi-wire set up.
Had a similar experience once (lost 1/2 the bass) with bi-wire went back to original jumpers and was happy again!
Decent jumpers allow both speaker terminals to look like a single electrical connection (almost) while with a bi-wire you are at the mercy of the "voicing" introduced by the additional wire. I am guessing in your case the speaker wire used is not the best match which became obvious in the bi-wire set up.
Had a similar experience once (lost 1/2 the bass) with bi-wire went back to original jumpers and was happy again!