The rack effects that are intended to run at line level generally have an input impedance too low for a Hi-Z guitar signal. However, I've noticed that the input preamps tend to have plenty of gain, so the guitars low amplitude is seldom an issue. At most just use a a buffer between your guitar and FX looper and you should be fine. Any Boss-type pedal in bypass mode, or an actual buffer should do the trick. Watch the output levels of the rack effects though, they can be pretty hot. The output impedance is also low, just like any good guitar pedal, so no worry on the output side other than watching the oputput level. If all of your effects and looper are true bypass, you will benefit by having a buffer pedal first in line anyway, in the case where everything is bypassed. It fixes this problem too - buffers are not as evil as some people think. It's Signal Processing 101. Good luck.