I've utilized an external ADC EQ before. There are a lot of them out their in the market as they are a 15+ year-old item. I just couldn't believe how just adding that EQ into the sound chain just trounced on the sound. While having the ability to tweak the sound can be a benefit, adding in the EQ was a step backwards. Lots of others will always recommend the parametric EQ, but there are some other options. I have the Squeezebox players in my house, so I have the option to use an EQ as a plug-in for the software. It's not a parametric EQ, but it's a bit more hybrid -- between the standard EQ and a parametric. Plus, you can configure the software EQ for how many bands you want to use. This option also supports streaming Internet music, too. Those options are also available as plug-ins for the Squeezebox software, making the opportunity to EQ your streaming music, too.
The HE-4 vs. the HE-5LE. A few of us have owned both, tend to agree that the top-end of the HE-4 is awfully like the HE-5LE. It is in the bass where the HE-4 steps back a bit from the HE-5LE. In fact, the HE-4 being discussed here was actually purchased by me originally (new) a bit earlier this year.
+1 EQ is a science and an art, and there are so many permutations and variations that they can certainly cause more damage than solve problems. I've been dealing with pro-audio EQ since about 1977, which is a totally different beast than headphone EQ. I live and die with parametric EQ on my pro bass rig- I cannot imagine gigging without it
However, once I have time I intend to spend A LOT of time with EQ on my various headphones. Taming treble bumps and smoothing bass are my primary targets. With my Grado's, I want to lower the bass bump to about 50 Hz, and roll-off that darn treble between 4000 and 7000 Hz