Oops, not 15kHz, but rather 15K ohms. The Deccas (and to a lesser extent the Londons) can benefit from a lower than standard 47K ohms input impedance, and a little extra capacitance.
In the late-1980’s I was graced with a phone call from Decca uber-enthusiast (and New York Audio Labs owner, as well as a very entertaining hi-fi writer) Harvey Rosenberg, in which he shared with me a lot of his wisdom regarding the idiosyncratic nature of the Decca design. At that time the changes made when John Wright took over management of the company (and changed the name to London) had yet to transpire, and sample-to-sample variability was notoriously inconsistent. Harvey advised me to load Deccas with an input impedance of anywhere from 10K ohms and up, and 200-300 pF of capacitance.
That variability was greatly reduced by John Wright, and London’s were made to a significantly higher standard than were the Deccas. John Wright retired a coupla years back, and the new owners are working on getting the company’s products back into production. I am very happy to already own a London Super Gold (with Decapod mounting system and Line Contact stylus) and a London Reference, neither of which would I sell at any price.