The bottom end is predictable. ~8K 6sn7 (260v, 9mA) internal impedance and 70-80H transformer primary inductance gives -3db around 20Hz. Actually I measured -2.37dB at 20Hz and -3.62 at 15Hz.
@alexberger Here's something to keep in mind. If you are not using feedback, then its likely at either end of the bandwidth of the amp that the FR will fall off on a 6dB slope. Filter theory tells us that a 6dB slope introduces phase shift to 10x or 1/10th the cutoff frequency. So a 20Hz cutoff will manifest phase shift to 200Hz since there is no feedback correction.
The ear perceives phase shift of a single frequency very poorly, but over a band of frequencies it interprets it as tonality. At high frequencies the ear also uses phase to sense the sound stage. A high frequency rolloff above the audio band can cause darkness in the presentation. At low frequencies a rolloff will be perceived as a loss of impact.
Transformers have bandwidth limits. The smaller you make the transformer, the wider its limits can be. So you have a chance using an interstage transformer to have one that has good LF response- 5Hz is a good place to be. Because you are designing an SET you're dealing with a low frequency bandwidth issue in the output already. All I'm saying here is the less phase shift you present to the output section, the less phase shift will be present in the output.
If you're planning to use a sub with this system this might not be much of a concern. But of you really want to use a sub properly, it should be cut off no higher than about 80Hz otherwise it will tend to draw attention to itself, requiring that it be in the same place as the main speakers. If a sub is used, probably best to drive it with a preamp signal rather than that of the amp due to this bandwidth issue.