Alright, I've completed the mod and have been listening to various music for about an hour now. All of this is based on what I remember my eggos sounding like as of yesterday as I hadn't worn them today until I did this mod. The overall results: I'm very happy with this mod! The bass on the eggos went from a level where it was clearly audible but had to be listened for to be appreciated to a level where it comes out at you but without overpowering the music. This is exactly what I like my bass to sound like: nice and tight and you can appreciate it without having to listen for it.
It sounds great on rock and pop. I listen to every kind of music though, and it sounds good on all of them including classical and jazz. It adapts to the mix very nicely: there's lots of bass when there needs to be and just enough where it doesn't. In terms of loudness compared to the rest of the mix, it seems just a tiny bit less than that of the PX100. Of course the eggos still don't have any thump to them like the aforementioned cans do, but I think they have a great balance now. I also want to add, and I very well may be imagining this, but the highs seem to be a bit less harsh then they were before doing this. Either way, the difference isn't very great, and if there is one, it's to the better.
The mod was very simple to do. About the only thing that took me any effort at all was to put the silver caps back on, accomplished by putting in one tab at a time, and squeezing them slightly inward. The hole that I made was a bit smaller than pea-sized, going by memory roughly 3 mm in diameter. The leakage seems to be a bit greater, I'd guess about 15-20% more, which really isn't very much considering what it was before. I never really thought the eggos were too great at preventing leakage when listened to from the side of the person wearing them and this doesn't make them much worse. I'm not sure how good a test of isolation this is, but they sound exactly as loud my music player as they sounded yesterday - that is, at the same volume.
I have to say doing this mod is very much worth it and makes for a more enjoyable listen (I don't know about balanced or neutral, but I like the bass coming out like it is right now more so than being in the background). Thanks to everyone for the advice. So yeah, my eggos just got that much better.
These are definitely my favourite headphones now and easily beat out other portables like the PX100 and PortaPros in terms of sound quality IMO. Now I just have to retrain my ears to not listen for the bass so much!
Bower: That's an interesting idea for gaining more isolation. I don't really feel like taking them apart and trying it though until I really need to, or someone else does first.
[edit for anyone going back to this thread for bass mod info]
First of all, I've increased the hole from several millimeters to about the size of a pea, same as everyone else (still not big enough to fit a pencil through as some people say.) This gives you a bit more bass than the original mod I did though not nearly as big a change as the original one was from stock.
The big thing that I did afterward was the cotton mod in addition to this hole mod. I opened up the cups by unscrewing the two screws under the padding (you don't need to take it off to do this) and putting two stretched out cotton balls in each cup so that the whole surface is covered. The result? While the hole mod increases the volume of the bass, this gives it a bit of a thump similar - though not quite on the same level - as my PX100s as well as increasing the isolation a fair bit (more so than stock IMO) so it's win-win, especially for those who don't want to solder a hole due to possible loss of isolation.
I'm now of the opinion that these two mods in conjunction with each other will allow you to harness the bass of the eggos to the point where one cannot complain about the lack of bass in any way. I think the cotton mod may make it slightly less tight although I'm not completely sure, but it's just a matter of whether you prefer the ultimate tightness of the bass or want it to have a bit of a thump. I think they groove a lot better now and while before doing this mod I was leaning toward using the PX100 for their groove, I'm not looking back anymore.
I've also tried doing the tissue mod where quarter sized pieces of tissue as put in front of the drivers. I don't recommend this mod as I think it makes them a bit too muddy even with just one layer, but some might like this warmer sound so give it a try if you want to. I think burn in is all that's needed to tame the highs.
[edit 2] Okay, this edit is way later, but not too long after the above, I took the cotton balls out. They actually really muddy up the sound. If you want to give it a try after the hole mod though, go right ahead, since it's hard to screw anything up and it's easily reversible (the only thing you really have to do is get the screws out and back in, and I think everyone can do that