So with a single-ended connection the 3B-SST may do fine with many tube amps.
I was going off my experience of the 3B-SST with a balanced connection (10k Ohm per leg) to a BAT 3ix. The output impedance of the BAT is spec'ed at 1000 Ohm, but is actually much higher according to JA's measurements.
I also found the following statement in the marketing writeup for the PS Audio Trio amp (50k input impedance):
"No tube preamp should be asked to drive an input impedance of less than 30K"
I was going off my experience of the 3B-SST with a balanced connection (10k Ohm per leg) to a BAT 3ix. The output impedance of the BAT is spec'ed at 1000 Ohm, but is actually much higher according to JA's measurements.
I also found the following statement in the marketing writeup for the PS Audio Trio amp (50k input impedance):
"No tube preamp should be asked to drive an input impedance of less than 30K"