Having said this, I heard many folks claim that electric-static headphones are super detailed and flexible with EQ. Now I start to feel maybe I should get a KOSS 95X and just apply a few db on the bass. What do you folks think?
'Stats definitely have their own sound. Could be the super low distortion, the very large drivers which are usually bigger than one's outer ear, or something else.
Even if you EQ up the bass in a 'stat and but the most powerful amp in the world behind is will still have less bass impact than an dynamic or planar. The 'stat's moving mass is far less than a dynamic or a planar so you have less vibration mechanically coupled to you head.
You'll definitely want some kind preamp to go with the Koss, either a DAC with a volume control or a headamp with a preout. Each gang on the volume pot is adjustable independently to act as a balance control, but there's no friction holding them together. This makes it essentially impossible to change the volume without through off the L/R balance.
Regarding fixing a headphone with EQ, there are a couple things to note about what can and cannot be fixed.
One is distortion/power handling. If a headphone stays clean at very high SPL then you have plenty of room to adjust it with EQ while listening at a normal level. Fortunately headphones usually do quite well at this, but since Amir started testing distortion at 3 different levels we now have a source to confirm.
The second thing to keep in mind is soundstage and imaging. These are
not easily fixable by simple EQ to a generic target curve. In the end, these factors still mostly boil down to a specific FR, but that FR needs to be individualized based on your anatomy. (Such as the shape of your ears and head.) This is fixable, but requires individualized measurements and more advanced DSP than just a PEQ.
Because of the
difficulty or
expense involved it may be better to just go for a headphone that already has great soundstaging (due to the driver's size and position relative to the ear) to begin with if that's what you are interested in.
What were you looking to get from upgrading? Better soundstaging, deeper and more impactful bass, or something else?