How high is "high impedance?" Xtal elements worked great with rigs that had 5 or more megaohm input impedance. Ham rigs with inputs down to around 1 meg used D104s. It's fine to replace a crapped out element with something else if the something else provides a rough match to whatever transmitter is used, and the xtal element is truly crapped out. As other stated, my experience is that if you want a good xtal element for a D104, you pretty much have to go to hamfests and buy D104s. Other elements don't work as well. I have a pair of those small Astatic lavalier mics--I bought them at a hamfest thinking they would have good elements and they did, but the size of the diaphragm is about half the area of a D104 element and they didn't sound all that great. You can go two ways at a hamfest: If you can't test the mic on the spot, look for bargains where you can pick one up for twenty dollars. I don't mind risking that on an untested mic., but $70 or more? No. Alternatively you can put together a small solid state amp and speaker that can run on a battery and a 1 meg input so you can connect the mic using clip leads and try it right there. Another method uses a battery powered solid state oscilloscope. A xtal element can drive the high impedance vertical input of an oscilloscope.