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

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? 

I had the BP6 as well and it’s a very neutral preamp but when paired with a Bryston 2B SST I found what you’re finding in that it was a little thin sounding.  But when I paired the BP6 with my McCormack DNA-0.5 RevA amp it was fuller and more natural sounding and much more balanced IMHO.  I’d recommend taking a look at this DNA-225 that’s a good value and should flesh things out very nicely.  You could also look at your DAC as upgrading to one of the newer R2R DACs might be more to your sonic preferences.  Hope this helps, and best of luck.

https://www.usaudiomart.com/details/649982529-mccormack-dna-225-power-amplifier-works-perfectly/

thank you for your reply sir.

1. room acoustics 

- 2'x4' absorption panel at front wall (behind of each speaker)

- 2'x4' diffuser of each side wall

- ceiling corner bass trap

- some acoustic panel at rear wall

- room size is rather small .. 10'x10'

- speaker pull out 1.5' from front wall and side wall, slight toe in

2. no power conditioner

3. bass is enough without sub

i have heco sub, but seldom use coz some song low freq may overblown in my small room

4. cables

- speaker cable use cardas quadlink 5c

- pre to power - interconnect nordost flatline black knight

- cd player to preamp - use chord clearway

- rega dac idling not in use