If your loop is tuned for the BC band, then it's no good for the shortwave bands. If you leave the loop tuned to a local BC band station, then it will overload the antenna circuits of your radio and give you the cross-modulation (aka "Bleed-through") that you describe when you want to listen to the SW bands. Most modern radio's perform really well on the attached whip antenna - the RF stages are "tuned" to work with this antenna. Some of them will disconnect the RF amplifier if you plug in an external antenna to reduce the chances of front-end overload.
Cheers - Martin ZL2MC - Palmerston North