Warm, fullbodied, smooth, and transparent

Depends on your budget: Transparent Reference Cables (particularly XL) from 10 years ago...not necessarily the new MM1 or MM2 which are a bit cleaner though still full sounding. Very rich, full but detailed and good bass in particular. i use them and really enjoy them...and while they are warm/full, i dont think many if anyone would say they are bloated or slow.

Purist Audio Dominus even more so. Also v expensive

Going to the budget end, the old Audioquest Emerald ICs were very musical, full and warm. not as detailed though...brand new they were $400...20 years ago. Probably could pick one up for 35-65?

good luck and pls keep us posted!

Hi. as you already know, the ones i've used, in order of increasing cost:

Audioquest Emerald ICs were very musical, full and warm. not as detailed though..

Cardas cross or golden cross

Transparent Reference Cables (particularly XL older cable
newer version MM1 or MM2

Purist Audio Dominus

they're all good and fit your description. if you need stuff that would be at the level of ARC Ref series/CJ GAT/etc, the latter 2 imho would be appropriate, and the latter 3 are all up to it.

if you are looking for stuff that would work with NAD, the AQ is very good (though of course better cables are still better).

Nice equipment...its worth spending the time you are, in order to get him the right IC. Given the Pass being (i recall) fairly detailed, smooth as a class A amp, and (i am guessing) the Cambridge is fairly straightforward...you'll want a cable that maximizes the delivery of information thru to the Pass...and maintains a fairly natural tonality. For value for money and excellent results, you may wish to try the new Sablon Audio Panatella...see the reviews on Six Moons, Jeff Day, etc. At their price, they will easily compete with and many would say better my old Transparent Reference at a third the price.