Looking at a pic of the Tecsun PL-660 with its back off, it seems to have a decently sized MW ferrite loopstick, along the top of the radio. It looks to be about 100-120 mm.
Still, I have a few radios with 100 mm or 120 mm loopsticks that technically should be hot MW performers, but in my hilly, low ground conductivity location they simply aren't. Perhaps that is your situation. Some areas are naturally better for MW performance than others.
Even a small MW loop will help. As the others suggested, you just set the radio near the external loop. You may have to move the radio from one side of the external loop to another side, for best performance. Or -- set the loop in front of, or behind the radio. ON the PL-660 the loopstick coil is top center of the radio -- that might be the strongest spot, but most radios work excellently if you set the radio to either side of the external loop, which usually is most convenient for tuning the MW band.
Most guys who DX the MW band use loops eventually. Very few radios are good enough to not use one -- and even those radios benefit from an external loop if you are in a low signals area.