For context, I own a Mojo and use it with BeyerDynamic DT770 Pro's (250ohm) and Denon D7000's.
I bought a USB jitterbug as I felt highs were a little "metallic". I think it helped a little bit (and only cost £40) but I confess I can't be bothered to switch backwards and forwards to compare.
I did last week discover that powering my mojo via USB3 on my Asus CH6 mobo, caused a very obviously hiss from the output which surprised (and disappointed) me. The problem went away completely when I switched to powering it via an Anker 5 port USB charger, hardly the last word in "pure hifi PSU".
I've not tried the Jcat stuff but my personal opinion is that I would very much doubt if the Jcat USB card would give you €435 worth of improvements on a mojo and there's a fair chance you'd hear no difference whatsoever or at best (also) "something" but you can't actually say what, or if it's better or worse. Don't forget that Jcat are the company that have a $1000 Ethernet cable for sale. Make of that what you will. Whatever merits their gear may (or may not) have, their gear is not for humble Mojo owners.
Be careful of being drawn into the world of audifools who always have an itch that they need to scratch. If you really want to hear a proper difference, put the money towards the next level up of headphones or even buy a different pair of phones. Some folks like to have different phones for different kinds of music or environments.
BTW, despite being a "man of science" I don't actually believe that ABX testing is always (100% of the time) a successful way to assess how much you are enjoying music from system A vs system B. ABX can obviously be a good way to help avoid you falling victim to expectation bias and blowing a load of money on a piece of flashy looking kit that makes "hard to put your finger on" differences. However I do feel that some positive changes are subtle and creep up on you over time. Also, switching backwards and forwards will eventually induces listener fatigue.
If you want something interesting to experiment with, download the free demo of Signalyst HQplayer 4 Desktop. The demo plays music for 30mins before closing down.
As a media player, its "ok" if a bit limited. I think it's written by 1 guy and you can see that the effort has gone into the back-end not the user interface. I use is in "network mode" and play music to it from my Roon music server.
It has very sophisticated upscaling algorithms to take the USB output up to 768Khz into your DAC. Mathematically it is (in terms of "taps") on the same level as the uber-expensive Chord Hugo M Upscaler and some claim is sounds just as good.
A license costs 238.39 GBP. I haven't pulled the plug on buying it yet but I'm close to doing so. It may have been the beer but I find it makes music somehow more enjoyable, natural and instruments more substantial and (dare I say it) more organic. It costs nothing to try.
