I've just been posting to them either on twitter or in youtube comments, using steemit for steemit.com and dtube dot video.
The people get millions of interactions so you can't be sure they've actually seen your comment, so I've left it on many of their videos.
The only one I've had a response from is Angry Pepe from Kek News Network who said he'd check it out. But imagine if just a couple of them made the move - it would start an earthquake.
I check Dtube nearly everyday and have seen the @corbettreport and @x22report are on there/here.
It would help if once the names are on here, they spread the word too.
So basically, just keep plugging. Maybe have a notepad template that you can copy and paste - just don't do it excessively. I would say one per video, so you don't get accused of spam, and don't include links, youtube doesn't like that you might recommend a site they don't approve of, and I'm pretty sure they won't approve of Dtube.
If you have a youtube account maybe make a review video to post on there.
Simple stuff, just be careful not to get flagged for anything.
RE: Introducing DTube: a decentralized video platform using STEEM and IPFS