The K-1000 is not impossible to drive - it's pretty easily driven. It was designed to run off a speaker amp. The beauty there is that you can use delicious DHT amps to run it. 300B and 2A3 tubes breathe magic into it. I especially love the 2A3. The catch is that you usually can't use those amps for other headphones. However, they lend themselves nicely to efficient singledrivers, horns or AMTs if you can find them.
You can drive a HD-800 nicely with the Gilmore Lite, though. I like the HD-800 on tubes, but the Gilmore Lite has plenty of power and is a very nice, clean-sounding amp.
You'll enjoy the HF-2. It's a strong performer, but the HD-800 gets a lot more head time in my rig. Almost all of the head time - I love those headphones.
I wasn't crazy about the Denons, but I want to give them a longer listen at the next meet. I don't think they're a good fit for my tastes, but a lot of people love them and I want to give them another chance.
I haven't heard the JH13. IEMs haven't done much for me, so I passed them over at CanJam. But the JH13 has gotten so many strong reviews I want to try them at RMAF. Like you, I worry about eventual degradation of the fit, that they can't be repaired (I'm not sure about that) and being able to resell them. Also, 95% of my listening is at home, so full-sized open headphones, tubes and speakers fit my listening habits well.
