Unbalanced pre out to balanced amp in

From the data sheet, a fairly wide range of impedances is shown for the load, but they also show that the output impedance of the transformer is 80 ohms. There is a rule of thumb that suggests that the load would be about 800 ohm (IOW 10X the output impedance).

There is an optimal value- it looks to me (based on the specs) as if this unit is designed to support 600 ohm operation. I bet 600 ohms would do the job nicely. Have you tried loading it ever?

With the right load not only will you get broader, flatter frequency response, but the unit will be more transparent too.

Is 'across' meant in-series or in-parallel?

By 'across' , I mean in parallel with the output.

If the input of the xformer is supposed to be 20K and the output 600ohms, then the xformer will express 20K to the input if there is a 600 ohm resistance across the secondary.

Its the same idea that you see in a conventional tube amplifier: if the tubes are supposed to have a 3000 ohm load, that will only be the case if the output (8 ohm tap for example) is loaded at 8 ohms.

You'll notice that in the test circuits that produced the graphs on the first page, that there is indeed a 600 ohm resistor (or a pair of 300 ohm units) loading the output of the transformer.

thx,
that's what I mean and sometimes use to half the resistance.