need advise, which is the weakest link in my audio chain

If you’re looking to upgrade components, two things…integrated like the Pass Int60 or even Int25, and new dac plus streamer. Ditch the bryston separates, use your Marantz as a transport plus stream from Qobuz or Tidal . Keep in mind, as I said earlier, that Marantz isn’t a harsh sounding player so the amplification upgrade will make a more positive impact thsn adding a dac to those Bryston components.

Few possibilities to improve…

1. Room acoustics - any room treatments? How big is the room? What does the speaker placement look like - distance from walls, distance between speakers, toe in?

2. Power - do you use a power conditioner?

3. Do the speakers produce enough bass in this room to balance out the high frequencies? If not, consider a sub or a change to floor standing full range speakers. 
4. cables - what speaker cables and interconnects are you using? 

Lots of good advice above…

I would add few things to try:

1. This will cost nothing: Swap diffusion and absorption panels - place the absorbers on side walls and diffusers behind speakers. If possible. Reason - room is most likely too small to make effective use of diffusers at the first reflection points. 
2. Not familiar with you amplifier and pre, but I would try another amp and if possible another preamp with it. 
3. I had the CD6006 it’s not a bad player but it isn’t as good as you can make it sound by adding a good dac. I agree with others here.

Just my thoughts 
 

@wturkey lol

@nagasakti70 just curious how thick are the absorption panels. You mentioned the sound becomes dull when you place the absorbers on the side walls. Also what is the material used in the panels? If it’s foam it would explain the effect. With quality absorption on side walls you should hear more focus and details in the soundstage. So something is off…

Yes the diffusion panels on the side walls leave a lot to be desired.
My recommendation still stands - absorption instead of diffusion on the side walls. Also, the corner bass traps up towards the ceiling…are they foam? They’re probably good at reducing echo but in my experience foam doesn’t do much for bass trapping and it tends to make the highs sound dry. 
I would start with removing the diffusors and placing absorbers there. Something like GIK 242 or 244 panels or something of similar quality and build including the materials used. 
This is a cheaper path forward. The more expensive approach is to replace your Bryston components with a good integrated amp. But the room issues will persist even with new and better amp. It will most likely be more tolerable. Again, DAC and cd player won’t get you there as quickly as the amp upgrade would.