Ladies and Gentlemen;
My TC Helicon units (VoiceSolo and Harmony G/XT) have only XLR outputs, but appear to be line level. . . . . . going directly into the Carvin's power amp section via the effects loop input, which is 1/4 inch. The signal should be hot enough as I've done it before with other boxes that have both 1/4" and headphone outputs. Do I need some impedance matcher thing to do this or will a simple Female XLR to male 1/4" cord do the trick?
The effect return is almost certainly an unbalanced input. The most common way to make a balanced XLR to unbalanced 1/4" cable is to connect pin 2 of the XLR to the tip of the 1/4" plug and both pins 1 and 3 to the sleeve. This works most of the time, but some electronically balanced outputs don't like to have pin 3 tied to ground (pin 1). This can cause distortion, which can be fixed by disconnecting Pin 3 and taking the signal between pins 1 and 2. If you buy an off-the-shelf adapter, get one that doesn't have molded-on plugs so you can re-wire it if necessary.