I can't say for sure how they compare but the DX was what I was originally looking for. Unfortunately I've never actually seen a Kindle DX in the wild and they're rare as rocking horse poo on ebay here in the UK.
The Kobo Elipsa is the first ebook reader I've used where I can read PDFs without zooming, which is a game changer for me.
In general I think DX or Elipsa sized screens are more useful for text books even in epub format.
It's much faster than my old Kindle but I was never really that bother about the old Kindle's page turn speed. The Elipsa also has much crisper text but again I thought my anchient Kindle was mostly good enough.
The Kobo search features are much better. My old Kindle's search was too greedy which made it pretty much useless as I couldn't narrow results. e.g. I couldn't search for "act" without getting results for "actor" as well.
The only irk I've had with Kobo is that the default PDF reader doesn't let you set a zoom level to reduce margins as you read. You can zoom in but you need to zoom out again to turn the page.
That's only a very minor issues though, I can still read things fine with the default margins. Installing KOreader or Plato was as simple as dragging a file onto a USB drive and give you every ereader feature you could want including margin stripping PDFs.
I considered a Remarkable 2 but I'm not really one for hand written notes and I wanted a frontlight, which the Elipsa has but the RM2 doesn't.
I actually wish I could have bought the Elipsa without the bundled case or pen as I don't use either. The handwriting to text or drawing to flowchart conversion is fun to play with but it's not something I need, I'm strictly using it as a reading device.
I'd guess that the Elipsa would be a good replacement for a Kindle DX but that's caviated by how you personally want to use it. There's something very pleasing about keeping an old ebook reader going though so it might be fun just to keep using your DXs until they absolutely die and then see what's on the market at the time.
Hope that helps.