I have owned BP26 and 4B-SST² since2010.
I pushed hard for recognition of the BP 26 as a World class preamp and finally Absolute Sound agreed.
When I replaced my Magnepan 3.6 with Magnepan 20.7 my dealer convinced me I did not need to also move up to a 4 cubed amp.
Instead he suggested a Marantz CD player. The $7K one I forget the model.
I also splurged on Kimber KS1116 and KS1016 wires. the run from pre to amp is $3,200 alone. well worth it to me.
Anyway after spending $20,000 more including thousands to get the Speaker resistors just right... I got stacks of wood, tuning.. bought piles of Furutech outlets. and moved stuff around.
To get the vocals to become coherent was the wood. Putting everything on wood and or exotic.The amp in particular I put up on a hard maple on old TipToes, top of Maple a Walnut butcher block and the instead of on the Amp feet amp resting on exotic hardwood bowl blanks.
The speakers themselves the large oval bases up on several 2x2 Bloodwood lengths. (to keep those tuned right and not dampened by the carpet they are up on 1/2 by 2" by1/4 maple bits.
Anyway after getting the right response for the room in general basically on. I used well known Female vocals same ones and repeatedly played them over and over to find the perfect toe in. When the voacals locked in as one clear sound I stopped messing around and measured those exact distances. SO the speakers can be placed precisely where I found them to be perfect.
Now several years late still very happy with the quality of the sound.
So the questions BP26 Good Hell yes.
Stay with 4B-SST² no problem
Improving sound with even more AC stuff. (Furutech duplex)better aftermarket AC cords, IMO tuning with exotic wood was my best answer. Started with Michael Greene's idea of tuning and took it in my own direction. Butcher blocks under everything various exotic wood bits under everything as feet some under stock feet better.. I did a lot of experimenting there. some butcher blocks doubled up. Best ones doubled up were Turntables and amp (surprising me was how amp was improved)
Anyway that is my two cents.
And finally seeking better vocal coherence was mainly toe in and very small adjustments to speaker position.