My present everyday set-up:
Modified K500 connected balanced (two plugs), a special trick of the anode/cathode output of the Audiotailor Jade. Terrific sound stage and airiness. GEC 6AS7G (or Tung Sol 5998) output tube, Telefunken ECC808 driver tube (with adapter).
The standard K500 has a 1/8-1/4 inch plug, and if you've lost the adapter I've found a replacement. This adapter doesn't have the usual thread, but a larger one. AKG original to the left, Audio-Technica replacement to the right:
Available from Thomann: