Jahap,
The Hi-line seems to be fairly well liked and is a good, quality option for connecting your digital source to the 252. I use one too and personally, I wouldn’t be spending any more to try and improve it at this point.
If you want to spend more then yes, you have plenty of options, and the XPS muted earlier is a sound suggestion. Personally, I would be looking at getting the fundamentals right if that’s not already been done. Mains quality is important, so a dedicated spur/radial or ring with its own consumer unit, installed correctly by a qualified electrician, can give some isolation from mains ‘noise’ created by other appliances in the house (this could be cheaper than you think - if you can accommodate it).
Also, for your NDX2, the digital network quality is important. If you have a few hours spare and are inclined to do so(!), have a look on the streaming forum at some of the threads which discuss the network and how various switches/leads can affect performance. Take some of it with a pinch of salt because it seems what works for one, doesn’t necessarily for another, but my point is you can try stuff cheaply or cost free and sometimes get quite amazing results. Having recently read some of this, I simply removed one of the two switches setup on my network (a cheap gigabit switch originally bought from Maplin) and simply plugged everything into the one remaining quality switch (Cisco 2960) - nothing more. The results were quite profound and considerably preferable to me - it was clear that the cheaper quality switch was somehow degrading the quality of the network (but that’s for another discussion!)
That’s not to say you will find exactly the same, but it’s worth a try and a cheap Cisco 2960 (circa £50 second-hand) has been found by many to bring decent gains over a normal cheap quality switch.
I would say that once mains, supports and network have been addressed as best you can, then you can rest assured you are getting the best out of your existing system and go on to find even clearer results when you change a cable or add/swap a box (if that’s what you want to do).
Oh and if you’re not already aware, keep your 252/supercap turned on all the time for best results; it’s designed to stay switched on and will take a few days to reach best performance from cold. I would only shut it down when you are due to be away or it won’t get used for a few days.
Anyway, that’s enough of my ramblings, go away and enjoy it!