Hi Jonnoshore,
Martin is the expert in this field, and he knows how an extra component added to the circuit here and there can affect things like roll-off at the top and bottom ends of the frequency range, and the DC offset at the speaker terminals. The nice thing about these LM3886 amps is that you can tweak them very simply.
I don't know about a 250, but I compared one of mine to a 140; same room, same speakers, etc. Without a pre-amp, with just a volume pot, the LM3886 was a bit different from the 140, but not necessarily better or worse. Then I connected it downstream from the NAC72 pre-amp and it sounded very similar to the 140, only very slight differences that I cannot really describe. Maybe a bit "sharper" and "colder" than the 140, but not that much in it at all.