Pryso, You (and I) were correct the first time. Jperry confirmed that the 501 is indeed an MC type. Other than that, your advice as an owner should help the OP most. Yes, not much need to worry about capacitance with an MC. The OP needs to be sure he is using the hi-gain setting, and then to check the load R adjustment. Possibly he is loading down the Shelter too much; that is to say that the selectable load R is too low in value. This would result in an apparent lack of gain (because energy is being lost to ground) and reduced treble, bloated bass.
Etnier, you may want to remove the cover of your unit and look for the adjusters that Pryso described. Then make certain that the load Resistance is no lower than 125 ohms, as Jperry uses for his 901. If you're not happy with that, increase load R progressively until you get better tonal balance and more apparent output from the Threshold. Or, more scientifically, find out the internal resistance of the 501 from the spec sheet. Start then with a load resistance that is at least 5X the internal resistance, and go up from there.