Why do all Mahler recordings seem too bright?

I can understand that to some degree.  You might try one of these:

Stenz/Koln on Oehms

Vanska/Minnesota on BIS

Fischer/BFO on Channel

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. 

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.

lowrider57 wrote:

May I go off-topic for a moment? I find many of the Bruckner digital recordings bright, and this only applies to the horns. Massed strings sound perfect, but all those mic's on the brass section ruin some performances for me.

Have you heard the Ivan Fischer/BFO Bruckner 7 on Channel Classics?

lowrider57 wrote:  

No, but I've heard the outstanding sonics of his Maher cycle. Will check it out on Qobuz.

The close and multi-micd Bruckner is not always what I consider bright, but the brass section can also be too forward in the mix. They're the loudest instruments and there's no need to emphasize them.

Well, try the second movement to hear what I think of as a perfect balance in the horns in the multichannel version.  (It's even better in a different, unreleased rendering that  I have the privilege to possess!)

@kr4 I wasn’t addressing brightness in the Ivan Fisher recordings, should have separated the two thoughts. My issue is with many modern day Bruckner digital recordings. There is emphasis on the brass (trumpets and trombones) due to over mic’ing or in the mix. The recordings are not balanced in a ff or fff passage. Adagios are usually well mixed.

I was not addressing brightness in the Fischer recordings of Mahler but giving this as an example of what I think is a well-balanced Bruckner recording. 

The reason I mention Bruckner is due to the large scale orchestra close in size to Mahler. I’ve seen quite a few Bruckner performances where 5 trumpets and 5 trombones are used. That's a lot of microphones being used for recordings.

Why do you corelate the number of instruments with the the number of microphones?

lowrider57 wrote:  Because some engineers in the heyday of multi mic'ing would place a mic on each individual instrument instead of 2 or 3 per section. Some conductors, particularly Karajan liked to be involved in engineering the production process. He wanted everything micd. 

Yeah, many used to do that.  Less common now and, strangely, even less  (relatively speaking) with multichannel recordings.  The ones that sound best in 5 channels used 5 mics, perhaps with a spot 1-2.