Im not sure how this would compare with the one your thinking of with the high pass filter. This youtube clip might help.https://www.youtube.com/watch?v=UVBDHSINSxQ
Up until now I have tried an off the shelf "listening" device ive seen rebranded and sold online as a safe listening device. I bought mine on Ebay. This wasnt very good.
I made my own prototypes piazo mics, some good some not so. I tried a couple of guitar practice amps, they worked good, but I wanted smaller more portable, and low power useage. These use 2 AAA battries, and can be bought for £20 ish second hand off ebay. So not a lot of money.
They work good, very light and portable, and they are designed for a quality sound reproduction, for serious musical practice.
The only other modification Im intending to make to this set up for now, was suggested to me by Capt Dunk.
His idea is to put an in line push button to contact switch. So you dont get all the crunching, and grinding background noises you dont want. Just push the button when your ready, and listen at the critical moment.
This is a good idea that I will make soon.
The Mic im running into this is very sensitive, which may be great if its stuck to a metal door, but if it falls off your lap when your practicing dialing in the van waiting for the kids, it almost bursts your ears.