Just FWIW the Threshold 4000 is a fantasic amp,although a wee bit long in the tooth by now, unless it has been serviced. The 4000 was not designed with tube electronics in mind and over the many years dealing with Threshold products I have never heard a 4000 work well with a tube preamp. It was designed for the Threshold NS10 and SL 10 preamps. Although the Threshold Fet 1 and Fet 2 preamps will work quite nicely with the 4000. The Nikko pre you have is holding back the 4000, so a better preamp is indeed called for. My last 4000 was paired with a Klyne SK5A preamp and it was spectacular sonically. One of my worst decisions was to sell the 4000 and the Klyne. I now have Spectral gear and it is as good if not a tad better than the Threshold, but I gotta tell ya the 4000 and Klyne set up was very special indeed.
If you do want to use the Cary just make sure you can return it if it doesn't work out for you. I do have my doubts on that based upon folks having gone that route. But you just might luck out and find a match.
As far as speakers go I am not a fan of the Proac line, just my opinion, many folks love them. I use Alon speakers and have found them to my cup of tea and as always others will disagree. Speakers are a matter of personal taste. You do have to audition speakers to hear for yourself if that is the signature that pleases your ears and musical preference.
If you do want to use the Cary just make sure you can return it if it doesn't work out for you. I do have my doubts on that based upon folks having gone that route. But you just might luck out and find a match.
As far as speakers go I am not a fan of the Proac line, just my opinion, many folks love them. I use Alon speakers and have found them to my cup of tea and as always others will disagree. Speakers are a matter of personal taste. You do have to audition speakers to hear for yourself if that is the signature that pleases your ears and musical preference.