My TI1's been like this since I got it (second hand)
Outputs 2LR & 3LR are absolutely fine, Output 1LR has a crackling noise like yours Sebastian, kind of like digital clipping; and now you mention it, it does seem to be on patches with more bass going on.
I thought at first it might be on loud patches but it still happened when I turned the patch vol way down.
A few years ago I posted my TI to Germany to the official European service centre (it was well out of warranty period) & the dude there said he couldn't reproduce the crackling & it was working fine (I even told him which patches I know for sure crackled).
When I got it back surprise surprise it was still doing the crackling (on Output 1LR only, just like before), although he did fix the other 3 or 4 things wrong with it.
So I just continued to use Output 2LR all the time, which is slightly annoying when swapping between single & multi mode.
It is weird that outputs 2 & 3 are fine though.
I think I tried the SPDIF output which mirrors Output 1LR, and it also did the crackling thing (as far as I can remember...it was a few years ago).
So that's probably no help but at least you're not alone...
Only now I payed attention to your last sentence regarding the cracking in the SPDIF output. That's a shame. That was one of the tests I wanted to conduct. I was hoping that it's a D/A converter issue. I'll will still give it a try on of these days. In other words, the only way to avoid it is through the usb-output, or the other Output (2+3)LR. I've found out another thing. I just bought a ModWave. I have it connected with a balanced cables on the input of the Polar. (Don't ask me, I think the Polar is so sensitive when it comes to its connections), Output 1LR runs balanced to my RME card. The crackling is still there. To me it is related to Output 1LR. How I wish it was as clean as the headphone output. Then you really hear the old Polar shining dam, impressive white box. I am going to make some more music now and see how it all evolves. . But there is some buzzing with lower frequencies. I wonder if you can route the input on the Polar to outputs Output 2+3LR.