Looking for $20k system for Mahler No 6

I assume you are looking for dynamic speakers that will not sound congested but will remain effortless on Mahlers 6th, in which case I would suggest a bigger Active ATC. If you have systems already for chamber music and vocals then simply use the source and preamp from this setup and use the active speakers with built in amp packs. I have seen some used ATC SCM 150's for sale for less than 20K on audiogon. Just be aware that these may overpower a 20 by 20 foot room....but at least you won't be limited in SPL or dynamics...I can assure you that you will still have plenty of headroom even at deafening levels. (ATC's main feature is effortless "Hi-fi" sound at very loud and dynamic levels with extremely low distortion...think audiophile quality sound at SPL's where other good sounding speakers would have long been blown up)

I think the frequency response of the speakers will have to be fairly good at 40Hz, maybe below for the "hammer" and bells. I think I would start there; reviewing the various graphs in Stereophile, UHF etc.

you should try the clayton amps. They are awesome and will give you the sound that everyone loves from tubes and the reliability of solid state.

My blatantly biased suggestion...

Speakers - Dynaudio C4's ($9K used)
Amp - Plinius SA 102 ($2.8K used)
Pre - Aesthetix Calypso... w/good NOS tubes ($3.3K used)
Digital - choose from Audio Aero, Esoteric, Accuphase, Ayre ($4K used)
Room Treatments from GIK Acoustics ($1K new)

I have no affiliation with any of these companies... just a happy customer.

The C4's really are great with large scale orchestral. Your room should be large enough to accomodate them, but I strongly recommend adding room treatments (bass traps & panels), as I recently found out they make a huge difference for very little money.

Well when you check out my system you will see I am highly biased when I pick the following components. But I listen to lots of classical and it sounds nice on my system, and I would think this would do it for you. All these but the Krell are in the used forums now, that Krell amp comes up fairly often.
Esoteric X-03SE 4100.00
Ayre K-1XE 4400.00 OBO
Krell FPB 600 (around $5.5K used)
Thiel CS7.2 5500.00 OBO
Anyway we all know what opinions are like, and this is mine. I would find every shop within reason of where you live and start auditioning everything you can. Have fun.