The reason they are terminated in a 3.5mm plug is because I've always previously used them with a mini Sound Professionals preamp into a Sony PCM-D50's line input.
I just recently got the Zoom H6 and I have noticed that the "Mic/Line" input is not nearly as quiet as the D50. When I used the preamp before, I would turn the gain up all the way on the D50 and use the knob on the preamp to control my levels.
However on the H6, if I turn the gain all the way up there is considerable noise, while the D50 was completely silent. I do not believe the mini jack on the H6 is a true "line in". So I wanted to try the combo inputs to see if they were less noisy.
Is there any solution to get this to work outside of re-terminating the microphones? Would it be ok ff I turn the gain up all the way on the preamp instead and use the Zoom's gain dial? I've always been under the impression it should be the other way around.
There's really no such thing as a "true" line-in in the sense that there is nothing between the input and the ADC. There's always going to be some sort of isolation be it op-amp, transformer, discrete amp, or combinations thereof. Ideally, you have the least noise at the lowest gain (gain here may even be negative in the lowest position) so you want to set the least gain on your noisiest section. I'm not familiar with the SP pre-amp, but I would assume a dedicated mic pre would be quieter than any gain section in a portable recorder and adjust accordingly (minimum gain on the Zoom H6 and control levels with the outboard pre-amp).
As far as AT853's, my research has run into multiple opinions and I have no definitive answer on how much voltage they should use. That said, my opinion is that they probably work best at about 5 or 6 volts. They seem to be OK with as much as 12, but I don't think more voltage helps any. PIP power specs for portable devices are all over the board. Best to actually measure it with a voltmeter with the mics plugged in. You may need to build a breakout cable to do this. My AT853's are the original Audio Technica version and have balanced 3-wire mini-xlr terminators. I use them with the supplied AT8531 battery boxes fed with 15V phantom to the XLR connectors. The AT8531's drop the voltage to about 4.5 volts no matter if you feed them 48V or something lower (15V is the other setting on my SD-MP-2). If you use the battery box alone, they only get 1.5 volts. I've never tried them at a loud rock show with this low voltage, so I have no idea how they work.