Sorry, only just found time to pull my el-8c out and plug both cans into the Hugo and do some comparing
I am glad you asked, because it made me take another good look & listen with the el-8s and realize they are better than what I had remembered them to be, sounds weird I guess.
Build quality - both headphones are top notch (remember that the 99s are less than half the price of the el-8s) and that's saying something. I consider the el-8s among the best looking headphones I have crossed paths with. The ear pads on the Audezes are a tad better than those on the 99s.
Comfort - the 99s are by far more comfortable than the el-8c. They sit lighter on the head while the el-8 press down at the top somewhat. While the earcups on the 99s are small and this bothers some people, I just don't feel this once the music plays - they sort of get out of the way. However I have used both cans maybe an hour at a stretch tops. The isolation on the 99s is also better than the el8 despite the latter being truly over ear with large sized cups.
Driving the cans - the 99s are far easier to drive than the el-8s which is unsurprising I guess. On the Hugo I never get past green on the volume level with the 99s while I start at blue with the el-8. While listening to 192 files of good recordings I found myself occasionally dialing up to purple (yikes) for the el-8 while the 99s still sounded good at green.
Sound - I tried out various music played out of my ip6+ via Hugo and using the Onkyo HF player and Spotify. Diana Krall, Amber Rubarth, Massive Attack, Beck, Ane Brun, Robin Schulz The short of it is that the el-8 has bigger sound stage and instrument separation and a more neutral tonality than the 99. This difference is stark with good recordings more than the usual compressed stuff. The 99s on the other hand are warm sounding, slightly cramped sound stage BUT play low frequency notes with authority that the el8s lack. Don't get me wrong, the el-8s do bass too.. but the slam is missing on the el-8s while on the 99s its there. When trying out low freq test tones the 99s showed more authority than the el-8s especially 300hz and below.
Hope this helps