I think we clarified by PM that the Melz 1578 does not need adapters.
BTW, two other 6SN7's that enjoy "holy grail" status are the Tung-sol 6SN7GT black plates (>$500 pair if you can find a NOS matched pair) and the RCA red base JAN 5962 (~300/pair- easier to find). (The 5692 is the ruggedized, low microphonics, low noise, high reliability, long-life version of the 6SN7GT). Some say it's the best sounding 6SN7, but again, the way these tubes are used in the Horizon, who the hell knows? Should be fun to play with.