There's definitely a sweet spot for gear when it feels like it plateaus, but if the headphone and gear pairs well it should always sound great when you sit down and take a listen. My biggest recommendation is to you is to ensure you are investing in some great tracks as the music can make the biggest difference once you have a good setup. There are a few users out there that do not like the EF5 amp and I'm not a big fan of tubes. I have yet to hear one that does not add noticeably distortion to a track. Sometimes it takes a while to find a track that reveals the flaws of a tube amp, but very few perform better than solid state gear. You deal with tube wear, warm up time, and added distortion. Some combos do sound fun like the HD800 and Bottlehead Crack with Speedball, but in the end this setup is altering the headphone's sound signature with the gear behind it. To me an amp and DAC should get out of the way and you tailor the speaker, in this case headphone, to your taste.
I do have a few questions.
1.) What DAC are you using?
Currently I'm just using my cell phone as first step was to get the headphones now I'm onto getting a proper source
2.) What pads do you have on the 5LE headphones? Take a pic and post it please and make sure the pic shows the inside of the earcup and outer pad. I'm hoping we can upgrade the pads with the Focus A pads.
3.) What is the best sounding track you have? Please post a link preferably on YouTube so we can take a listen and give you some feedback.
Lately when I've wanted to see what a speaker/headphone can do I would play the following
If I remember correctly the upper midrange on the 5LE sounds a bit uneven so my guess is you are looking for some more detail?
I would say more detail and seperation, some tracks are better then others but I find everything is all close in.
Here are three tracks I really enjoy you should try with your setup.
Artist: Ben Prestage
Track: 2:19
Album: Down-Home & Home-Made
Artist: Karl Latham, Ryan Carniaux, Mark Egan & Nick Rolfe
Track: The Dull Flame of Desire
Album: Constellations
Artist: Entheogenic
Track: Urubus
Album: Flight of the Urubus
Those tracks should open your ears a bit to what the next level of music can sound like. That is if you are not already at this level.
Cheers!