The 250DR will be brilliant. You are making wise choices. The ND5XS2 is a great product, especially with a different DAC. I got one as I wanted the latest Naim streaming platform and wanted to retain my NDS as DAC. It sounds fantastic.
Your nDAC will sound great with a decent power supply. Even an XPS2 will give a great uplift.
When I went from Nait 5Si and CD5Si to 82 HiCap 250CB the uplift was huge. I loved my 250CB. The trouble with me is I like playing music loud and found that the 250 cut out when played loud for continuous periods. It has an auto cut out when it gets too warm! So this was reason for 135s.
250DR is different beast to 250CB and will be really good.
I have had a pair of NAP135s for over thirty years, and have never yet got them to cut out. I suspect that I’m too afraid of frying the plastic membranes inside my ESL57s!
My 135s have never cut out only the 250!
Actually, Dan, I have an NAC250 in my ‘big’ system too, and that has never turned itself off either. But I daren’t turn up the volume control past the twelve o’clock position.
I have never managed to do that, with my ex-CB 250… 
135’s have fans for a reason… 
The speakers were very efficient Focal Aria 936s at the time and was using a streamer so was about 9.30 o’clock playing drum n bass and techno for several hours. If you play rock, classic, pop or jazz you don’t have the sustained bass assault which was probably causing it. I was playing as loud as possible before it clipped the speakers!
Have calmed down now slightly! Was a particular phase I went through. The Focal Sopra 2s are much more resistant and louder! 500 beckons.
But seriously, a 250 is bit weak for my demands. The 135s have been perfect for me.
I would put the NAP135s alongside the (original) ARO at the very top of the list of the best things to come out of Naim HQ.
That’s why I haven’t parted with them Graham. I have changed most things out apart from NDS, Supercap 2 and 135s. I love them. Great amps. I had them serviced in 2019. They are hard to beat and have history. I think it’s special to have old things that work really well. Get them serviced Graham. You’ll be in for a treat. Speak to Darran at Class A.
As hard as I’ve tried, I’ve yet to get the fans going in my 300 DR……
There are advantages to always having had speakers which are smaller - and perhaps more … efficient… 
My speakers are extremely effecient.
Fans are going now on 135s! Loving it. Listening to Teckno1 radio.
“I think it’s special to have old things that work really well “ that what I keep telling my good lady😉
The 250 input socket is XLR, which you might think would be balanced as that is the usual socket used for balanced connections. In this case, Naim use it for an unbalanced connection in all 250s with the exception of the New Classic version.
I don’t think I’ve ever managed to get my fans going.
the beauty of having 4x 135s spread the load 
My fans migrate over winter. They’ll be back around June/July I expect. 
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