Why do all Mahler recordings seem too bright?

Thanks to @kr4 - for recommending some that are better.

I don't necessarily regard them as "better."  I just recommended them in the hope that they might seem "better" for you. 

Mahler included funeral marches in his symphonies and also use a large number of brass instruments. So I am not surprised some poorly done recording sounding "bright." However, there are many Mahler recordings CDs and LPs that sounds well balanced from top to bottom without any edginess or harshness.Some of the DGG recordings can sound bright, but not limited to just Mahler's work.

Symphony #4 Klemperer and Philharmonia Orchestra (EMI), Symphony #5 by Solti and CSO (London, Super Analog), Symphony #6Horenstein and Stockholm Philharmonic Orchetra  LPs are highly recommended. They sound well balanced in my system with Thiel 3.6 speakers, Mark Levinson 23.5 amp, Cary slp 03 preamp, and Audio Research PH5 phono amp. Cary and AR has NOS tubes. Another gem is Mahler's 1st symphony with newly discovered "Blumine" movement recorded by Frank Brieff and New Haven Symphony Orchestra (Columbia Odyssey).  I am in Colorado and have attended Colorado Mahlerfest few times. So I have a very good idea how these works sounds live.

There is nothing bright about the recent Los Angeles recording of Mahler 9. Or the CSO Resound recoding of Mahler 3. Both are the best recordings of those works played better than any other orchestra. 

Mahler = Dynamic= the more power the better

Jasonbourne52 Spot on

Mahler included funeral marches in his symphonies and also use a large number of brass instruments. So I am not surprised some poorly done recording sounding "bright." However, there are many Mahler recordings CDs and LPs that sounds well balanced from top to bottom without any edginess or harshness.Some of the DGG recordings can sound bright, but not limited to just Mahler's work.

Since you said you're listening to Hires, you're either listening to CD or streaming digital. I wanted to expand on the mention of DG recordings. DG was way behind other record labels when converting from analogue to digital. Known for their multi-mic and close-mic'd techniques, early digital sounded bright, sometimes harsh, especially the brass section. Some 1980's CDs nearly make my ears bleed, and I have a well dialed-in system. There was good advice above to try some other recordings. A good quality master is key to reproducing classical or any music using acoustic instruments accurately and realistically.

Eventually, DG digital improved but I wouldn't consider it audiophile quality. I've found MTT recordings to be high quality, very listenable, even with their high-end extension. 

I wouldn't start changing your components, although cables do make a difference and you said yours are new and may need more run-in time. I think you should check for any vibration coming from your system. Components need to be isolated from the shelves they sit upon. Are you using any anti-vibration footers or devices? I listen to classical almost exclusively and couldn't get rid of the brightness until I removed vibration from my system. Vibration from the DAC and CDP contribute most to poor reproduction of digital. 

There are many high quality classical recordings, keep in mind early digital may have problems. I love old recordings of the great maestros and their orchestras. But sometimes I want to hear high quality so I seek out record labels and orchestras which have been recorded and mastered well.