The omni thing depends a ton on context obviously.
(There’s a lot of optimization you can do too, using something like @tremblap’s HISSTools. I know for a lot of his live sampling oriented pieces he will do some measurements at the venue and correct for the microphone and the speakers in the hall to do things like getting a “realistic” omni distance mic from a close direction mic etc… I do a much lower tech version of this where I sort of ‘bleed’ out the room and dial out problematic frequencies. (I’d like to eventually have an automated system that will sweep and tune a filter to minimize feedback spikes)))
For the value the k&ks are hard to beat. They also have lots of variations (hotspot, twin spot etc…). As far as preamps, contact mics have specific impedance requirements, and a regular mic/mixer preamp isn’t ideal. Cable length also matters a lot too. I would recommend making something like this, which does nothing but correct the impedance (a buffer). I have a couple of these sitting in a drawer somewhere.
What’s even better though is something like this:
(@mods, do let me know if it’s not cool having a schematic like this up here)
This one is a buffer and a preamp, and has a built in EQ to boot. I found the schematic online then had someone make a stripboard layout that fits inside an altoid tin. This sounds fantastic. Using this gives you a ton more bottom end without doing any boosting, just by buffering the signal. You’ll notice a big difference even with a shit contact mic.
