I found the 99's to be a bit bloated in the bass with the larger pads - fixed that with EQ on the ES100 - which controlled the bass and improved the sound stage by bringing the vocals more forward. Absolutely love the combo - and while not quite as open/airy as the HD6XX, i strongly prefer the 99/ES100 combo for most of my listening. I also found the balanced cables improved the sound stage and provided slightly better imaging and spacing.
That said, I did order the smaller pads from Meze and just installed them last night, as I left my es100 at the office - my initial impression is that the pad difference was very noticeable - way more than I expected, actually. It toned down the boominess and improved the mid range - similar, actually, to what I get with the EQ from the ES100 and larger pads.
Didn't listen long, but those were my initial impressions.
Also like to play around with the Schitt Vali and have fun swapping out tubes - I do notice a little more warmth and 'silkiness', especially in female voices depending on the tube - I hated it with the stock tube!
In terms of the ES100, sound quality is absolutely stellar and much better than from my phone - LG G8 - which has a pretty decent reputation for audio quality - without question, the ES100 is worth every penny - IMHO...