I collected the KRX3 from our local Parcel Farce depot and after 3 nights (2 hour sessions), had it in the K3 and working. The build went very smoothly with all parts present and some extra screws and washers for good measure. Instructions were well written and left nothing to experimentation.
However, I did make one silly mistake where I put the filters in the wrong positions (L-R not R-L) and ended up having to take the KRX3 back out and reposition them. It took me an hour of head scratching and foul language till I realised my mistake :-) (the KRX3 filters are unusually numbered from 5-1 (L to R) so your widest filter goes in far right).
I've configured AUX RX to come from the supplied BNC connector as I don't have a KAT3 installed and I want to be able listen to separate bands. True diversity rx is a show stopper. I have an 80m doublet and a 40m quarter wave vertical antenna and was listening to someone in the EU calling for and working JAs. The JAs were masked on the doublet by EU muppets calling back, but the vertical still allowed me to hear the JAs in my right ear through all the EU QRM in my left ear!
Experience with the K3 main rx has taught me that I only need 2.7KHz tx/rx (5 pole) and 2.1KHz rx (8 pole) roofing filters to cover all modes in the sub rx. This is partly due to a design flaw in the K3 which automatically selects the narrowest roofing filter for the selected DSP bandwidth. If there was a manual option, I would buy and fit more roofing filters on the basis that 'I' could choose when to use them. i.e. The K3 selects a high loss narrow roofing filter (500/400/250/200, if installed) and then tries to amplify the output back to the original input level, just because I selected a lossless DSP filter bandwidth of anything at or under 500Hz. If you don't fit these filters it has to stop with the narrowest roofing filter fitted (2.1KHz in my case). This means less internally generated noise and no QRM difference in certain band conditions, a win-win in my book. Of course there are band conditions which suit having a tight roofing filter, but I'd like to be the judge of that!
Anyway, the usefulness of the K3 has skyrocketed with the addition of the 2nd rx and makes my MkV even more redundant than it was. I guess I'll have to think about selling it on before it dies of lack of use.