The EZ Amp I have no experience with, it just looks like the same unit that is sold in eBay/Alibaba, just with a magnet added. If there is a high pass filter, I don't know.
I just went with the Lockmasters Pro-Amp. A bit pricey, but works great. Very sensitive to high frequency noise such as metal to metal contact, and can get the volume from nearly zero to your-ears-pop. This is a simple amplifier made from LM324 op-amp and a handful of discrete components and the box is size of a deck of cards. The headphones that come with it have mono/stereo setting and volume control per side and are extremely comfortable to wear for hours due to their padding and the strap is adjustable.
The only huge minus with the Pro-Amp is its case. It is the cheapest hard plastic case which is a tad small for the headphones and the surface looks like those cheap automotive tools you find from dollar stores. There is no place for anything inside, everything is just thrown in and close the lid.
I went bought myself a Pelican case for this, now everything sits nicely, fits all my other tools, has a nice label on it and nothing clangs around the box
I also added couple of magnets on the amplifier box to have it attach to the safe door, as on normal safes one can put it on top of the safe and hope it doesn't fall, but on a vault door there is no shelf for it. If you put it in your pocket, you will hear your shirt rubbing the cables in your ears.
TL;DR: Get the Pro-Amp, but get a new case for it from somewhere.