Alright I got my C5 yesterday and now have put some hours into it. My chain is Spotify 320kbps -> FiiO E18 DAC -> C5 -> SZ1000 with HM5 leather pads.
So the bass is much more well controlled and does not bleed into the mids as much now. The C5 bass boost is MUCH better than the E18's bass boost in that it doesn't cause distortion or as much midbass bloat.
The SZ1000 do not clip anymore but I can still get distortion if I have the C5 turned up too high with EQ+High Gain+Bass Boost. Please note I use a ridiculous amount of EQ, though. Here is my EQ graph:
As you can see this is not the best way to EQ something, but it's how I like it to sound! The impact is great (could use a little more still to be honest. I think the Taction Kannon ruined me!) and the other frequencies are not drowned out too badly.
All in all the C5 was worth the price and is leagues better than the E18. I highly recommend it.