The Heathkit DX-40 first appears in Radio-TV News Heath ads in January 1958 priced at $64.95. It replaced the similar but somewhat less powerful DX-35 which appeared in the ads from July 1956 to December 1957. Designed for the 80 to 10 meter ham bands (including 11 meters), it uses stacked B+ for the oscillator and buffer tubes (both 6CL6). Its final RF tube is a 6146 for 75 watts plate power input on CW. Screen modulated AM operation is rated at 60 watts input peak. A switch on the rear panel selects one of three crystals or the jack for an optional external VFO. A typical VFO is the Heath VF-1. An accessory power socket for a VFO is provided on the rear panel of the DX-40.
Try this link for Heath catalog info on the DX-40.