If you assume that a higher-current triode has a lower plate resistance, then it may matter very little how much current you pull. Using very round numbers:
0mA - 90H
1mA - 65H = 65mAH
2mA - 43H = 86mAH
4mA - 22H = 88mAH
10mA - 7H = 70mAH
20mA - 5H = 100mAH
40mA - 3H = 120mAH
Or: 1mA in 12AX7 gives 60K Rp, 40mA in 6BX7 gives maybe 2K Rp. 60K and 65H is 159Hz, 2K and 3H is 106Hz. Not the same, but awful similar for two very different tubes and currents.
Also very low-fi.
And +4dBm out still isn't "pro". That's what the VU reads; peaks will be +20dBm. If we know performance degrades slowly, we may cite performance at +4dBm and let the peaks be a little bent; if performance goes all to crap at +6dBm, it may be annoying.
You have a lovely test-rig that measures some ideal property, maybe; but there's really only one way to prove this pudding. Put it in an audio amp.
Nearly all transformer-coupled tube amps have to be considered as POWER amps. That means peak current will approach bias current. What peak current? Say +20dBm in 600Ω, 18mA peak. Round up to 20mA, transform 4:1 to the primary: 5mA. Load the transformer, either 600 on the secondary or 10K across the primary. Drive your BJT at 5mA DC with 4.5mA peak AC current, 45V peak. Use an emitter resistor well over 100 ohms, keep the base bias/drive impedance well below 10K, like near 1K (or even the 90Ω of your H-P oscillator).
Sweep down from 2KHz. Either the sine will drop in level, or the sine will distort bad. If it distorts, note that and reduce the AC level to maybe 1mA peak, keep going lower. You will get the envelope of either small-signal droop or power bandwidth, whichever is more limiting.
You will get slightly different results if both sides of the transformer are terminated. Small-signal response will extend lower, but also at lower level. Since un-terminated connections are common, I'd get those numbers first. Then you can try adding shunt resistance to see what it takes to flatten the bass, but added shunt resistance will reduce peak output.
If we could get a very low plate resistance at 5mA, we could do better. But 12AU7 or 6DJ8 gives around 6K Rp at 5mA... at 16H this works out to 60Hz unterminated, 38Hz terminated.
I think you have a choice of no-DC or low-fi. Is this "Class X" iron any better in no-DC use than their generic iron?