My system's high frequencies hurt my ears

Try toeing the speakers in until the axis of the speaker crosses well in front of your head. Some of the excess highs might be from the first reflections from the side walls. This will also change the back wave quite a bit and could lower high frequency energy off the back wall as the sound waves have to travel further to reach you.

FWIW, it is not a straight forward speaker issue no matter how logical that may seem due to metal tweeters reputation. I have had similar problems with silk domes etc. Spend a lot of time positioning and repositioning your speakers before you decide to replace them. That is, at least the cheapest alternative. You might also consider trying, as suggested, deadening somewhat the rear wall.

FWIW, going back to your speaker wiring and I/C's. Nordost gets a lot of sales based on how it presents the high frequencies. Great clarity, at least so it seems. For a while. But what I think occurs is that the highs are not in balance with the mids and lows making the sound appear bright. I worshipped mine for about 3 months. Try a cheap experiment - get some Canare 4S11 Star Quad speaker cable and some Blue Jeans I/C's and use them as a benchmark for other brands - they are really inexpensive and good. They have a large following for just that reason. If after repositioning your speakers and treating the walls behind your speakers and using this wire you still have high frequency problems then perhaps it is time to move on. Unfortunately, as you may find out as many others have, this may not be a solution to your problem and that is why I emphasize further exploration with what you have.

I have tried Blue Haven and it is nice but it is quite bright and slightly etched so consider another cable. Agree with pointing speakers away from towards you. Try straight forward. Beyond that, soft dome tweeter speakers -- perhaps Dynaudio or Dali.

I don't buy the all digital front end argument. I've heard several non fatiguing all digital, computer-audio-based front ends, including my own that sound fantastic. The highs in my system I would describe as crystalline, extended, natural, sweet. Far from forward, harsh, edgy or fatiguing.

The suggestions about speaker and cable synergies and room treatment strike me as the most informed guesses.

Robpriore, I think you have a simple problem which is caused by the Nordost Blue Heaven and a hard dome speaker. At the best of times, whenever I have heard Blue Heaven it has a bright coloration that is very obvious when you listen to some of the better Nordost cables up the line. Add this propensity for brightness to a hard metallic dome based speaker and you have a recipe for what you are describing, IMHO.

Some feel Nordost speaker cables have a tipped-up frequency balance so you might want to try other cables. Acoustic Zen cables are one brand I have had good luck with but there are others with a less tipped-up high frequency balance.