I would think bx4(v5i opamp) with n3pro LO is more warmer and bigger soundstage compare just n3pro alone.But not major difference. Just a step higher only. But that what i feel everyone hear difference and preference.
My BX4 on stock op amp...
A18t on O-module
using Violin solo with orchestra background for testing...
BX4 = 12year scotch (with blinking and interference)
N3Pro = 15year (in ultraline mode)
=18year (in Triode mode)
N3Pro ULmode LO BX4 = still a 15Yr but with noise
DX200+Amp9 = 21year scotch but with a layer of veil
N8 = 25year scotch, expansive and not as intimate as the other 3
So different modes using different tubes..
Not really one better than the others.