Get UERM and use EQ. Seriously.

CEE TEE has posted some good commentary about them over at the site that shall not be named.

The tl;dr, if I recall correctly, is that the two are entirely different animals. The PRM at stock settings is by no means flat and is not intended to be flat. Moreover, it is unlikely that one could ever tune it to be as flat as the UERM. To vaguely paraphrase what I remember him saying: 'it is NOT a 5 driver UE IERM'. So, if the UERM is something you are strongly considering (because of certain widely agreed properties), then the PRM is probably something you don't want to consider, and vice versa.

Unless you really know what you are doing (good ears, knowledge of EQ, know recordings, have a lot of time), messing with the three knobs is more likely to result in a sound you won't like. Heck, I probably have more golden ears (in terms of knowing relative EQ and tonal balance) than 99% of others out there, and I probably wouldn't trust myself, that is without the aid of measurements. (the subjective AND objective are both necessary in my book.) Let Tyll's experience with the PRM be a word of caution: http://www.innerfidelity.com/content/totl-madness-24-top-line-custom-ear-monitors-reviewed-tyll-hertsens

Still, it doesn't mean one wouldn't be able to get the results they wanted.

Purrin - Where do you EQ the UERM? I'd like to play around with them a little.

I'm interested in that too . I remember reading something about Purrin lowering 2.5khz or 3.0khz by -3.0db. I could be wrong though, and it could have been for another headphone or even said by another person; I've been doing too much reading here the past 2 days. 

Gotcha. I'll give that a try and play around a bit with it from there.