You'd have better luck with a HF or UHF approach (13.56, 433 MHz) and going active, especially since it sounds like you are gunning beyond even a couple meters. To get the most range out of your LF device, you might consider using large, well-tuned loops that are free from metal in the neighborhood (which you probably already have done). Having multiple orthogonal antennas/tags can be helpful if your application doesn't guarantee fixed orientation. Also, having a high power, high sensitivity reader gets you extra distance:
http://www.ti.com/lit/sg/spat178b/spat178b.pdf