I've been toying around with my OK1 bud finding ways to tweak the sound without relying heavily on EQing.  I started applying some passive techniques to attempt better or different results from stock.
 
The OK1 is an interesting earbud.  It is Yuin's top model and an excellent sounding earphone by any measurement.  The design starts with a high quality driver that produces quite remarkable sound.  It is encased in a rather generic package, but it does hide some technical aspects under it's plain shell.  Without actually tearing one apart, I can only speculate on its overall design.  The enclosure seems to be a mix of an aperiodic enclosure and a ported enclosure.  Behind the the driver there are two foam lined ports that add breathability to the enclosure.  There also is a long port that runs along the cable shaft that exists at the base of the earphone.  I've toyed with blocking these vents with pieces of tape to toy with the sound, and they largely seem to act as they appear.  If you completely block the rear venting, the earphone gets very constrained in movement and low frequency output.  Partial blocking to various degrees modifies the sound signature and can tighten up the bass line at the expense of bottom end sensitivity.  The port acts largely as a port with added bottom end response open or earlier smooth roll off closed.  The vents seem to affect a little higher in the frequency range, even up into the midrange.
 
 
The two slot ports behind the woofer with a foam lining.

 
 
Single port:

 
My take on simple vent tuning of the earphone.  I'm simply using small pieces of electrical tape to partially cover the rear venting and the port.

 
 
Additional tuning comes from the front side of the earphone.  This earphone by default is a little bright.  It is a broad, smooth peak, but the higher mids and treble are a dominant feature of the presentation.  I looked for some simply solutions to balance out the sound.  The main solution was to simply add cotton to the rubber earphone cover.  I sacrificed a couple Q-tips to aid in my experiment.  The object was simple, add enough cotton to the point where the overall frequency response began to appear pretty well balanced.
 
Adding cotton to the stock perforated covers.  I currently have one Q-tip (both sides) per cover, and this is a pretty good starting point.  I will toy with more later as one and a half to two may yield a slightly warm presentation.

 
The OK1 with the cover on and cotton filled.  The cotton strands are long enough and the perforations small enough to not have issues with cotton sticking out.

 
 
The last step I toyed with was the front end of the earphone.  The OK1 incorporates two parts to the face.  It has a large center opening that is screened and used for the IEM conversion tubes, and it has circumferential holes around the perimeter.  The center is open straight to the driver which can be easily viewed, and the surrounding holes allow other passages for sound to escape.  What I find with this earphone is the overall sound is spacious yet imprecise.  My thinking is that the overall sound has a large area of escape and there is an issue with the lack of a pinpoint source.  I started toying with blocking the center hole and blocking the surrounding holes with tape to see what I got in terms of sound.
 
Sound hole modification, testing blocking the outer holes.

 
 
So what has all of this toying gotten me.  Well, the results are interesting but are following expectations.  The rear venting has a large effect on the movement efficiency of the driver.  If you block too much, it can get rather constrained and peaky.  One gain with partial coverage of the rear venting is that the bass does get tighter and more forward, but the gains are in the high bass and lower midrange with a loss of bottom end sensitivity.  I have come to like blocking of the smaller slot vent as it does help emphasize the upper low end and low midrange enough to better match the well presented midrange of this earphone, and you get a more forward and even presentation.  There is a slight tendency for stock to sound slightly loose/aloof.  Blocking the large vent slot takes it a step further and makes for a rather forward midrange and lower midrange with good punch and energy.  It is however on the edge of being a little too constrained for the woofer in the lower frequencies, and the lower end does really start to sound constrained.  The lower port acts largely as a port should.  If you block it, you lose some bottom end.  I've toyed with it but find it's better just to leave it open.  With the bottom end already being light on sensitivity, I do find all this effort to be a bit wasted without counter effort to the high end.  Yuin has made an effort to squeeze out a good amount of low end sensitivity and presence from a midrange and high frequency specific driver.  The stock tuning shows that they made a great effort in getting as much low end out of it as possible.  I do see gains to be had with covering the shorter slot vent, but that's about it.
 
The front side has been largely beneficial.  I've seen great results with the cotton in taming down the top end.  The one Q-tip per cover is a pretty good start in terms of changing the emphasis from the upper midrange and treble down towards the midrange.  I've upped it to 1.5 Q-tips of cotton batting and think the end response is pretty well balanced.  The sacrifice is some of the top end detail and edge does get lost or a better term, weakened.  I feel the trade off is a better one.  There could be some effort to carefully choosing the density of the cotton to shape the response some.  I do want to say that adding cotton batting like this does decrease the overall sensitivity of the earphone quite a bit.  You're blocking a lot of the higher frequencies, and the volume setting does need to be set higher, and wattage requirements go up.
 
The hole covering is interesting.  Blocking either doesn't drastically change the end sound signature.  Rather blocking a part is more so an exercise in presentation.  I'll break it down into two ways.  Having only the outer holes is beneficial in the sense of ambiance.  It adds spaciousness to the sound.  However, the presentation is less direct and pinpoint.  Blocking the outside holes takes away some of that space but also adds some precision to the sound.  I'll note the changes aren't massive but are perceivable.
 
Current setting: 1.5 Q-tips cotton batting, outer output holes blocked, and toying with partial coverage of both rear vents.

Interesting... I don't think I'd ever try the cotton because I have a feeling it'll distort the clarity. But, then again, I'm only an enthusiast and by no means a technical guy, so my opinion should be taken with a spoonful of salt

Figured I'd update this a little bit.
 
I did eventually go back to no batting because there is a significant enough loss of detail doing so.  There's also a bit of dispersion caused by the batting and random spread of sound waves which makes imaging suffer.  In the end, I kind of see it as something one can do if they must.
 
I did some more toying around with the IEM mode, not too much, but I took a look at the frequency response of the earphone in one IEM setup.  It's kind of interesting the scale of change using these earphones between earbud and IEM modes.  For the response test I took the long IEM tube (helps move the bud out a ways to ease fitment, and I paired it with a medium bi-flange tip.  I continue to cover the surrounding holes around the center opening.  In bud form, this helps with imaging.  In IEM form I just do it to limit sound leak.
 
Here is a comparisons of frequency response between both configurations.  To get these graphs I run a pink noise track and EQ the earphone to (ear) flat.  This ends up being my perception of the response, not absolute but relative to what I hear.
 

 
This is of course rather significant in change.  It shows some of the challenge in trying to get this earphone to sound as good as it can be.  This is a rather hard earphone to bring to its best because it pretty much requires a powerful amp and powerful EQing.  The driver used is very good.  The response just isn't ideal.  While treble can be addressed through filtering, there isn't much one can do in IEM form to reshape the response passively.  With EQing, either turn out quite well.  I almost never used the IEM format because there isn't much I can do outside of dedicated processing.  The bub format is bright but likable and has a lot of air and a very direct sound.  The IEM format is bassy and muffled in the midrange.  Once balanced out, the IEM format is quite good, natural with an authoritative low end (yay for a big driver) and a good top end that is detailed and balance.  The most noticeable aspect in the IEM form is the dynamic range of the low end, but that's what a big driver gets you.  It digs low and is fast.  Low frequency notes are clean and effortless.  It's perhaps a little less open/airy in IEM format, trading a little top end presence for bottom end presence and making a little more intimate presentation.  I kind of want to say warm, but that's kind of a tonality thing.  There's a little less transparency because you feel the earphone vibrating more than in bud form which pulls your attention to it some, but this is a small change.  The bass is stupid quick on this thing.  The driver is aggressive and dynamic and once it's piped right into the ear it gains extension and authority too. 
 
I haven't really toyed around with nozzle lengths and tips much to see if something can be done with that midrange dip or adjust the bass some.  The midrange may just be a lost cause in an IEM format (remember Etymotic's comments on this?).