Without getting too complicated here, I'd suggest a slight variation of my original idea:
Leave the existing output alone. Add a second output using a MOSFET (selected for drain-source voltage 100V+ greater than your B+; and minimal input capacitance) as a follower (source, instead of cathode). Connect the drain to B+; the gate to the plate of the gain stage, with a protection diode from drain to gate, then a big resistor to ground, from the source. Select the resistor for 10mA or so. Source connects to a series capacitor (your new 10uF cap) the new output jack.
Virtually zero load on the triode (and BTW, the 6BQ7 is a very nice sounding tube!), new output has no effect on the existing circuit, so long as the power supply can handle another 20mA or so.
The existing cap and it's sonic signature are left in place. The new output coupling cap can be compromised a bit if needed to control cost, since this output is intended to drive a bass amp only. Metallized polypropylene should be just fine, just avoid Solen Fast Caps.
In the picture attached, you will have BOTH outputs. One, a triode, to your high frequency amp, and one with the MOSFET to your bass amp.
On the subject of gain pots: none are perfect, good ones are expensive (P&G or autoformers), or difficult to implement LED optocouplers). If you don't need one, don't add one. Crap ones, and even good ones WILL degrade your system sound, it's just a matter of degree.
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