There are a few interesting things to try.

First the minimum reactance PS. It will require a CCS to execute. Let's try two CCS as active plate loads and do a simple 6CA4>>L>>100uF Oiler/motor run.

One or two red LED in series will do a fine job of cathode bias. we are running absolutely constant-current through the valve.

Simpler PS, and Cascoded DN2540N5 current regulators have a low output mu-follower output. These two benefits ought to be enough to justify building a second CCS. 400V D-S. Full wave from a 150-0-150 plate winding will keep things safe 'til the cathodes heat up, and the PS turns properly regulated L-C.

Choke from Hammond, 30 Hy/40 mA. PS cap, eBay's midorimeadows has ASC 370 VAC/100 uF for $10.

On to coupling caps. 5-10 uF motor run oilers are good here. No sense making output z too high at low frequency.

Here is the extreme example of where this design can be taken if you need some time to relax, and building is the chosen method. This one is L-C to a pair of CCS, feeding a pair of VR150's for each channel, which is actively loaded 2C50.

It sounds better than it looks, it only took a weekend to build.
regards,
Douglas

....damn Thrintun!