Why do all Mahler recordings seem too bright?

I also stand by my other comment that many recordings are brighter than they ought to be. Treble energy dissipates in the concert hall. However, the way orchestras are recorded - with a mic at most stands - does not recreate the balance heard mid hall. The mics are capturing what the musician might have heard, but not the audience.

There are (at least) two issues here. First, you are right that treble energy dissipates in the concert hall and, consequently, a good recording must capture that balance and the associated ambiance in order to have any expectation of reproducing the sound realistically in the home. Very few today (for many reasons) are created the way you describe and that’s a good thing.

Second, too many home systems are configured to create a flat response at the listening position without dealing with room acoustics and speaker-room interaction and that can result in a brighter sound than is desirable.

Since you said you’re listening to Hires, you’re either listening to CD or streaming digital.

Not necessarily. I listen mostly to high-resolution files and, whenever possible, in multichannel, as well. That includes the ones I have already mentioned along with a goodly number of direct live mic feeds. In some cases, I can compare the live feed with the commercial release. High-frequency imbalance is rare.

I’m going to place my bet that the issue is a combo of nature of the recording combined with more distortion from amp during louder or more challenging sections of Mahler for example when the pipe organ sucks the amp dry or louder brass sections. I will be surprised if it’s a vibration thing but you never know until you try. Can be multiple factors like that combined and any improvement might help. Power conditioning would be the other area that is worth having a handle on for smoother sound and less fatigue. Also physical isolation of digital components and power amps from the rest is always worth a try. No ICs running alongside power cords? Little details like that.

Listen to the Ben Zander/ Boston Philharmonic Youth Orchestra recordings of the 6th and the 9th.  Recorded live in Boston Symphony Hall, with, to my ears, no EQ or compression.  Multi miked but mixed to a very naturalistic stereo.  Sounds great on my speakers and my headphones.  The same forces just recorded the 4th and last year performed the 1st.  I am waiting eagerly for the CDs.  I attended all these concerts from a mid-orchestra floor seat.  The kids are allright!  Maestro Z has these young performers rehearsed far more than pro orchestras can, and their skill and enthusiasm are evident in the music they make.

It's well known that in the early days of digital recording and DDD CD's, DGG had some of the worst nails-on-chalkboard digital glare.  But as time went on things improved, and like I say, more recent DGG Mahler sounds fine to me.

I don't think anybody has mentioned treating the room acoustics.

@mgrif104  do you have any room treatments? You should be using some type of absorbion to reduce standing waves. These are the mid to high frequencies and can cause music to sound bright.

And I reread your comment

it’s harsh on the top end when the dynamics are loud. I’m pretty sure it’s not my amp clipping though it sounds like it.  

When I've experienced brightness or harshness in the music it's at all volume levels, not only at loud dynamic peaks. If this is the case, @mapman may be right about your amp clipping. It's being overdriven at at loud volume.