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

thank you everybody to give so many good suggestions.

1. yes, my listening room is given me lot of trouble when initial setup

beside too small, worst it is in square. but that the only room i have at this moment.

i have tried many acoustics placement.

my experience if absorption panel at side wall result the top end roll off and it sound lifeless too.

so long term plan renovate house, i have an option to extend the room from 10'x10'  to 10'x18'

2. any idea which combination may give more positive impact ?

- solid state preamp (keep my bp6) + tube power amp

- tube preamp + lexicon nt-212 (keep my current amp)

- Sugden A21SE class A integrated amplifier 

3. does chinese dac any good for my upgrade ?

Nagasaki, A few suggestions:

(1) As several have already said, your  room is a box and is not optimal in dimensions. Please take the advice offered and play sound with your sound treatments

(2) Your DAC situation is confusing at best, and a great DAC is key to all of your digital audio. A great DAC that fits the exact sound signature you want would be a Denafrips Pontus II 12th Anniversary…..about $1,850 through Vinshine Audio

Unless or until your room issues can be addressed, component upgrades will most probably be money wasted.

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.