> get the frequency response to not roll off below 300Hz.
You noticed that too.
There is a trick of using a steep bass-cut between the vari-gain stage and the next stage, to reduce thump and clipping-time. This is compensated with a bass-boost somewhere else.
But I don't see any bass-boost, so I agree: this is a communication speech processor. Perhaps useful for many musical chores, lead instruments and voices.
I wonder if it will be stable with less bass-cut. But surely it didn't motorboat in the original use. So something has gone bad.
> Not sure why you think 6BA6 will be better as it is usually only good as a Mu tube when run as a triode
6BA6 was intended for use as a pentode in variable-gain amplifiers. Warm-up your grandmother's kitchen radio, turn from a strong local station to a weak distant station, it nearly equalizes the audio levels (not exact, but 40dB into 10dB). 6BA6 is much better than a 6AU6 when you need to reduce gain 30 or 40dB. This is practical and necessary in radio reception. There are real problems with putting 30+dB gain reduction on good audio.
Both triode and pentode modes are possible. Pentode has an advantage that gain reduction does not involve a significant change in plate resistance: all practical loads are "low" compared to a pentode plate. In a triode, as current and Gm is reduced to reduce gain, Rp goes up which tends to increase gain. Triodes must be loaded in very low impedance (with low output), very high impedance (higher than lowest-current Rp, which is usually impractical), or swung over a VERY wide range of current to get sufficient GR (a popular plan). One advantage of the triode, with resistor loading, is that minor tube mismatch tends to self-correct as Rp interacts with the plate resistor and supply voltage.
In this case: while 6BA6 might be better in some way, I have to believe the Old Men knew their stuff and picked 6AU6 for some very good reason. There was a lot of collected wisdom in the old radio-labs, and plenty of test-gear to keep them honest. It sure was not cost: at the time, 6BA6 was more common and probably cheaper than 6AU6. EVERY 5-tube radio had 6BA6 or equivalent; 6AU6 had wide use but not like 6BA6.