Welcome Pillo917.
A Chord 2 x RCA Phono to DIN4 cable should be fine for the physical connection.
As for Naim being “tricky and delicate” it’s not that so much that - Naim amps are extremely well built and generally very robust - as that Naim amps are designed in a particular way and usually best match Naim pre-amps as they like a bandwidth limited signal (gently rolling off around 40kHz) to work from. Ultra wide bandwidth pre-amps can potentially upset a Naim amp. The amps also don’t have extra inductance networks on their output (this sounds much better) instead using a rather elegant solution of relying on the speaker cable for the necessary inductance for overall stability. This does mean you need to be careful with the speaker cable that you use - it needs to be low in capacitance and moderately high in inductance and of least 3.5m per channel in length (longer here usually sounds better, up to 10m+). I see you have NACA5 which is perfect, so long as it’s more than 3.5m per channel in length and well terminated.
You can read more about this in the FAQ;
The Threshold is a rare bird here in the UK. I do recall the Stasis power amps being well regarded back in the day though and I did hear one such system with a Stasis amp sounding very good into Magneplanar IIIs when I was in Boston during the '80s. Is it an ultra wide bandwidth design? You may not know for sure, but I guess it’s easy to find out how well the Threshold and Naim amp pair up by giving them a listen.
By the way, it’s a fairly old bit of kit I would guess? Do you know when it was last serviced? That will likely make a substantial difference to performance.