There are some similarities in planar and electrostatic sound since they both work with diaphragm. Just don't expect planar bass since the diaphragm are so much thinner/lighter weight and thus move less air. This seems to be, from what I've read, exaggerated on the Monoprice (I only have an ESP/95X).
The HE-400 is a criminally underrated headphone. I don't know what pads you are using but I absolutely
love my HE-400 with Lambskin Ori pads (or AliExpress knock offs, which is what I use
https://a.aliexpress.com/_dWNgVgi). It widens the Soundstage and gives them such a rich and detailed midrange which is not something that hifimans are known for. I liked them before but now they are in tight competition for my favorite headphone in my collection. It's a shame they don't make them anymore. I'm really glad I didn't sell them when I got my 4xx.