ok, it's been a while since I got advice here! I have now changed strings, and here's my verdict
I discussed this with Mark Gollihur here, and also got advice from Pirastro directly in Germany. The strings I had on were actually the NS Design Traditional, which to my knowledge are identical to D'Addario Helicores (just shorter to fit the NS Design string length).
I ended up ordering a set of Pirastro Perpetual Solo strings, with a Passione high C-string to compliment it. The strings feel a bit softer (The Solo set is intended for tuning F#-A, rather than E-G, hence, tuning it as a standard set they will feel quite a bit softer under the fingers).
There is a quite different feel to these strings than the previous "NSD Traditionals", especially for the D, G and C strings. The C-string in particular (Passione vs NSD C-string) sounds softer and more "in-line" with the other strings. The degree of "mwah" is obviously dependent on the string height, which is also the acoustic tone of the instrument (lower strings increases the mwah and decreases the thump). In this respect, these strings are not too different from the NSD Trads (Helicores) for the E&A strings. But I like the D-C-strings a lot better now.
So, ultimately.... are these strings similar to the Pirastro Evah Pirazzi Weichs that are on my upright?
That's a hard question to answer, since the host instrument is so different. The Evah Weichs are definitely softer, and requires the action to be set slightly higher to find a good balance between string noise and tone. The Perpetual Solos are a little stiffer, which allows the string action to be set lower.
Both are very musical strings. Does my NSD sound like my upright? No, it doesn't. Does it sound closer to it? Yes, it does.
Am I happy with the string change on the NSD? Yes, definitely. In particular, the G and C strings are much more in line the other strings. I didn't like the NSD TRad C-string at all, but with the Passione C-string, it sounds like an upright string. I think I could possibly have lived with the NSD Trads on the E&A strings, but for the future, I will stay with the Pirastro Perpetuals on my NSD.
One note of caution, though. The E&A strings are quite difficult to string on the small NSD tuners. I struggled getting them onto the tuning peg, to be honest. The
E-string can not be shortened, otherwise the string will not fit inside the tuner! (Yes, I learned this the hard way, and had to get a new E-string.)
By being careful and guiding the string properly and slowly onto the tuner, it sort of works... but it's tedious! So be warned!
To be honest, the difference between the E&A strings on the NSD Traditionals and the Perpetuals are not enormous, so it could be an option to use NSD Trads on E&A, Pirastro Perpetual Solo on D&G, and the Passione C string.