Why do all Mahler recordings seem too bright?

Yes it’s always hard to say where a problem comes from with gear one is unfamiliar with but if brightness is persistent throughout the recording and not just the louder peaks that are the problem could just be a matter of some tweaking. Again hard to say. Power conditioning might help. Hard to say with absolute certainty. Do you have a smartphone? If so download a sound level app like decibel and use that to measure spls during peaks or other times the brightness occurs. That will help establish if the issue correlated to the spl peak levels or not

Adding a sub or two is almost always a strong option to consider with any monitor speakers, especially for large scale classical symphonic recordings. . They do not extend flat down to the lowest octave guaranteed. Filling that in alone adds balance and may helping . Using an active crossover to offload bass from mains to subs allows main amp to not have to work as hard which always lowers distortion.

Smaller monitors alone are never optimal for any large symphonic works. Ok perhaps, but not optimal.

Here is a good read about value of adding subs: https://ohmspeaker.com/news/for-optimum-sound-how-many-subs-and-where-do-you-put-em/

Btw I picked up the entire Bernstein Mahler symphony series on CD about a year back somewhere on eBay new for about $12. Best deal ever! Can vouch for fact that these recordings are not inherently bright. Streaming Symphony #2 now as we speak. This thread reminded me it’s been too long! I do rip all my CDs and stream them only for playback and I find that to be a very robust way to help assure quality at the source with my digital recordings

Yes well that was one of the big things about Ohm Fs ie being able to drive them well on the day. It’s a piece of cake these days with the newer Ohm Walsh models and newer amp technology like Class D.  
 

Oh right those are bigger Kefs right?  Should be up to the task with the right amplification I would think  

Btw offloading low end from mains to a sub can help with any speaker even large more full range ones because the bass is where an amp works exponentially harder with lower frequency and excluding the lower frequencies allows the amp to work much less harder which always results in lower distortion. So a powered sub or two with active crossover might still be just what the doctor ordered. Your 100w/ch amp would likely never break a sweat with a sub handling the lowest octave or two instead.

Huh? Wha? My many Mahler recordings are actually pretty well balanced, tone-wise. Maybe it's my forgiving system or my admittedly ancient LPs, but from the Solti London recordings to the Bruno Walter Odyssey recordings to the Horensteins to my three copies of the Vanguard Wunderhorn recording, they're all just fine.