Try Roon. It's EQ is very good and easy to use and just works. I've grown tired of fiddling with APO and having to enable or "install" things for each source every time I restart my computer it seems. I thought Peace and APO were good and full of every feature one could want - they are...but the reality is, it's not always the easiest.
I tried Roon out and noticed they have an EQ that can load convolution files and also has a nice graphical EQ with a good set of filters and control. Certainly not as many as APO, but plenty.
Unfortunately it leaves an EQ now at a monthly cost...but it also happens to be a really good player that better manages your library and helps you discover stuff if you're using Qobuz or Tidal. So it's not simply just for the EQ.
As for the differences in planars from Hifiman. You're likely hearing their house sound. You'd get a more dramatic difference between brands. Though you should get a small difference between the lineup within a brand too. Like others said, how much is it worth to you? It's subjective.
Don't get me wrong, you can't take the lowest end in a lineup and expect it to be close to the top end. They clearly have "better" but once you get past a certain threshold, it's diminishing returns.
In my opinion, it's not about "better" it's about "better for you." It's unfortunately a bit of a brutal trial and error process in finding what you like. Not many people are able to try all of the headphones that they are considering before buying. You have to be fortunate enough to live near a good hifi shop. Or you find an understanding retailer who doesn't mind you returning a bunch of times...but there's only so many you can try and return before you give up or they cut you off.
I've found head-fi to be excellent in gathering opinions from people. It helped me find a new pair of headphones that I absolutely love and never expected!