Naim was founded as an amplifier company, and many posts on the forum are full of knowledge about the various pre-amps and power amps.
I remember about a decade ago, I too was very keen to climb the amp ladder, thinking all digital sources, more or less sounded within a few percentage points of each other, nothing much to it. I was completely and utterly wrong !
I remember many advices, tell me I was wrong, I should focus on the source.
Over the years, I was lucky to hear all the Naim range, and many, many sources, LP12, CD555, and NDS555, NDAC, NDS, Dac V1, NDX, NDX2, all the Chord Dac range, all the Linn Streamers all the way to KDS… All of them superb at their price ranges.
and as Richard Dane wisely advises, putting money in the source is the way to gain maximum musical satisfaction
An amplifier only amplifies what the source gives it. Even a Naim amplifier. Even a 552/500. These amps should have no character of their own, and the improvement comes from less damage to the signal that originates at the source.
Information lost at the source, can simply never be regained at a later place in the chain…
Interesting to see what the OP will decide, having heard a 252/SCDR it is a very fine preamp, but will it make me replace my 282/HCDR… oh well… only after I spent more money improving my sources…I’m perfectly happy with my 282/HCDR for now.
These new Rega turntable upgrades do look to much more tempting to me, as well as the NDX2 as a source to my Chord DACs… as well as a source to play music through their own internal DAC.
And if I were the OP, I would upgrade the NDX to NDX2, that is where I would put my money.
I would also think of upgrading the 250.2 to 250 DR, as well think of a Naim phono stage for the Rega turntable.
I don’t myself particularly look for hi-fi effects from upgrades, or focus on frequency extremes, I only focus on getting the maximum possible recovery of the all important musical signal, engagement, and emotional connection to the music…