My July 2025 production Luxury+ also has the 86280-60891 amp, version 1400.
The bass does not appear to fade in and out, but the default frequency response and fader tuning is still pretty garbage, IMO. The 120-200hz-ish region of male voices is super over emphasized and "woof-y", like a poorly integrated, overpowered sub. I still get very little volume when fading the audio to the rear speakers.
My "okay enough, but not in love" tuning is to fade the audio to the rear two clicks, and turn down the treble by one click. I keep speed-sensitive volume and surround sound both on, because the speed sensitive volume seems to be very specifically tuned to the surround sound frequency response; bass and treble seem to get really oddly emphasized if I disable surround sound. It's a bummer, because I think surround sound worsens & disperses the pinpoint spatial imaging of vocalists.
Similar thoughts on my September-build Luxury+ that was delivered last week. I'm withholding too much judgement as I did experience a very similar acclimatization to my '17 GS350 ML system, with notable differences.
For reference, I'm a hi-fi enthusiast with multiple high-end stereo setups at home, my latest built around two floor-standing transmission-line speakers that are flat down to 25 Hz powered by a 300 watt Class-A hybrid amp.
While driving, I almost exclusively listen to high-bitrate MP3 or FLAC files from my Android phone via Bluetooth, using the wonderful Poweramp app. At first my '17 GS's ML system sounded insultingly thin. I had only been able to test drive a base-stereo equipped-GS prior. After finding similar threads online at the time, I discovered that disabling surround sound & speed sensitive volume, coupled with tuning the 3-band EQ (coupled with tweaking parameters in Poweramp) transformed it into one of the best car sound systems I've heard (and best I've owned). My favorite test track is the Peter Gabriel-Genesis song "The Chamber of 32 Doors" - the bass, among other details in this track, is incredible and in my GS it's quite the experience.
Back to the '25 GX - I've found like others that it is heavily front-biased and the liftgate-mounted sub is of little consequence as opposed to the sub in the GS. This is also due to the GX's lack of rear door woofers. Where my GS can withstand almost mazixing out the EQ bass, in the GX I'm experiencing distortion in the front-door woofers above 65% or so with 'indie' rock (from bands like the Charlatans that aren't terribly bass-heavy) - this is with the fader set to middle.
I'm keeping mids near 50% as the front mid drivers get a canned sound above that. This is the only way I can achieve a bit of a 'punchy' bass-emphasized sound that I prefer but it easily leads to overdriving the front door woofers which have to carry too much of the mid-bass load.
With time I'm growing accustomed to the sound profile of the GX ML but do find it a missed opportunity to provide a richer sound experience in such a premium vehicle. Like others have suggested, even just having a separate subwoofer level adjustment could provide a marked improvement. The biggest disappointment for me is that where my GS ML system can be significantly augmented by the output device EQ (my phone), the GX ML system is far less robust. I'm not sure why you'd tout the speaker count or system wattage when it is overall a fairly thin-sounding system lacking the fullness of many 'base' audio setups. Enthusiasts seeking those specs will hear the system's lack thereof.
I'll keep tweaking settings and hoping for the best.