Hi Don,
This is a tough subject, as what is nirvana to one is meh to another.
In order to address this, my recommendation is to set a budget, and make decisions based on that.
FWIW, I think your DAC is fine. If your present setup is a bit dull, would look at Beyerdynamic options. I would look at the reviews/ratings at Sonaworks to get a handle on the headphone performance. To be frank, most reviews for headphones are HIGHLY subjective, and frankly not worth much. A a given reviewer thinks about XYZ cans actually means very little to any of us in our individual environments.
The headphone amp CAN make a significant difference. If one looks at headphones 80 ohms or over, a good headphone amp can and does help. Case in point: The Audeze LCD-3 phones with a proper amp can easily rival the better Stax ESL setups. With a more pedestrian amp, you won't hear what the headphones are truly capable of (just like Stax).
However, something like a Audeze LCD-X at 20 ohms, can be driven by just about anything and sound good. They retail for 200.00 more than the Focal Clear, but IMHO, it's a better option (because I am hooked on the planar/ESL sound from headphones). The LCD-X does not need a special amp to sound great.
