Yaxi stpad2. They change the sound drastically. Higher mids&highs (from 1 kHz upwards) go up by few magnitudes and with it detail retrieval, airiness and everything associated with it. BUT also at a cost of too much edge between 1.5 - 4 kHz, which has to be tamed down, so it doesn't cause fatigue and introduce harshness into the sound. That's why EQ is needed for that.
I'm owning 99Cs for more than half a year and auditioning Empyreans for more than two weeks at home. I'm running them both from RME ADI-2 DAC, which I settled on after testing around 9 other DAC/AMP setups (portable, semi-portable, "chifi" stacks, etc.., ranging from $200-$600). And must say that even stock ADI-2 is miles ahead from everything other I tested so far, so a massive upgrade even for 99s (lesson learned - don't trust just the "measurements sect"
).
If I'd compare stock 99s and Empys, then Empys are absolutely for those who want almost neutral (still little warmy), natural, relaxing and fluidy music. And for those who want primarily emphasized (middle)bass and don't mind less detailed & less open sound, 99s are still a better option with my gear.
Thing is that 99s can be turned into absolutely different headphones, if you change everything surrounding the drivers (foam, pads, cable). And mine 99s doesn't sound like stock 99s at all - it's a complete redesigned sound, by hardware and also software means (by RME's EQ and other settings).
So if I compare my current 99s with stock Empys, well.. I was in a denial for about a week, because I so wanted to like and buy Empyreans, but then slowly came to conclusion that my 99s are for my taste as good or better in almost every regard. Especially for the highs - more vivid and sparkly. I also invited my girlfriend to have a comparison of those two for about an hour and she said the same.
But I knew that Empys are much more capable if provided by the right chain, so I ordered TOTL silver cable and gave it another chance. And yes - that cable delivered. With stock cable I didn't REALLY like the sound (not that it was bad by any means, I just knew better already), but with silver quality cable I started to love them. Currently after some extensive tweaking of EQ inside the RME for Empys as well, it's a very close call. I love them both equally. I'd say that the presentation of sound is different, 99 is more intense right inside my head, Empy is more relaxing, more natural and open sounding. I'd say it has a lot to do with driver placement from ears - drivers from 99s are almost touching my ears (with Yaxipads), whereas Empys have a very wide gap thanks to superthick earpads.
One thing to note is that with some higher-tier dedicated amp, Empys would probably scale much better than 99s, since I believe that my current chain is bottlenecked by the AMP section of the RME. But anyway, it's the best AMP I've heard by FAR.
