The Atma-Sphere has the output impedance you require, which needs to be at least ten times less than the amps input impedance. Too bad it's a boutique company though:-) CJ has been around a very long time.
@jtcf FWIW Dept.: Atma-Sphere will celebrate 50 years of operations next year, but perhaps you need it to be longer than that to be considered a 'very long time'?
The MP-1 was the first balanced tube preamplifier, introduced in 1989 and to this day is one of the very few that supports AES48 with the ability to drive 600 Ohms with ease. This means it (and our MP-3) can drive multiple amplifiers with no worries.