Naim NAP 250

Ye they sound little diffrently everytime.Go to Naim forum,there is alot of discussion about Naim 250.And yes it could be used with another preamps.

I have an old fabricated-case NAP250 (smooth panels screwed to frame - external transistors, 5-way binding posts)circa late 70's.
In the last 20 odd years its been through two lightning strikes that caused serious rebuilding.
The first was around 10 years in age and it was like getting a new amplifier. Cleaner, clearer, more subtle than with the aging power-supply/caps. Second was about five years after the first and didn't result in as much change.
So they at least sound different as they age.

I've never had an opportunity to drop a newer model NAP250 in my crude setup to really know what the differences are. I have heard the newer models in showrooms but didn't hear anything that couldn't be ascribed to the different setup.
From description I've read I would think the newer model is a bit better in bass - I went active with a Celestion System 6000, partly because the weakness of the bass in my unit.
Removing the lower freq from the NAP250 (sent to Bryston 4B-NRB) really helped the "air" and subtlety of the upper end - has nowhere near as much work to do.