DJ100? Never heard of it.
Ok, seriously...
I was just listening to it tonight for about 2 hours unamped using a tiny Iaudio 7. I NEVER do this. It actually wasn't too bad, but a small amp really helps improve the bass and basically everything.
I don't think any mods are required at all to make it sound good. Koss got most everything right when it comes to the sound. It's well balanced, but leans towards having a fun signature.
It's focus is on the mids, then the bass and then the treble. I kind of view it as a thinner sounding HD-598 that's much more aggressive with it's sound. Maybe I don't have a clue what I'm talking about.
I'd say it's balanced somewhat, but nowhere near like a studio monitor, but it does have a clear sound with a decent amount of detail. I actually find it much less forgiving and more detailed than the HD-598! Yes, really!
I do think the stock pads are not very good, so you can get the M50 pads for them to improve the soundstage, treble and bass. With stock pads the comfort isn't that great. Even MDR-V6 pads will improve them.
I don't think the K99 could be that bad. I owned the K44 and I imagine that MIGHT be similar. If so, I had to EQ down it's terrible bass.
K44 with an EQ is actually quite good, but I paid only $30 and it's comfortable.
BTW for burn-in on the DJ100, you will only need 3-5 days. If it sounds great out of the box just skip it. If it's not clear and has mids that are not forward, you need to burn it in some. SOME DJ100s sound HORRIBLE out of the box until after burn-in. Wish I was kidding.
Another nice alternative to the DJ100 is the KRK KNS-6400. They have some similarities but the KRK has much less bass impact for sure. The 6400 almost comes close to dethroning my DJ100 as budget king, but not quite!