The A400 was always an analytical amp, but not very musical. It's Phono input was below par as well.
I owned one for 3 years and realised that it needs neutral and controlled sources and speakers. Plus it's handling of deep bass was wayward.
Saying that though the B&Ws are nor a great speaker. Lack control in the low end, plus the top end is exposed due to the recessed midrange. Changing the amp won't undo their disjointed sound.
I eventually paired some Mission 780SEs, then the Rega Kytes. Both had differing presentations but they both helped mask the A400s incisive sound.
Long term though you'd be better served (if you are looking at 2nd hand) with an old Creek 4040s2, Naim Nait 3, or Rega Brio 3 (all had good phono stages), as I think they can all still be serviced at some point (essential to maintain performance).
Plus change the B&Ws. They will always impart their character so much that equipment changes won't be as apparent as they would be through more musical speakers.