The "white box" could be called Fixed Bias, all the current version of lf339 is using fixed bias method in the cathode circuit.
I bought my amp before it so my amp was self biased, if I got one definitely I would open it and check it out.
I bias the input stage tubes with resistor, RC bypass, LED and finally with the rechargeable cell. The first two methods are self biased the LED and the rechargeable battery is fixed bias.The advantage of the RC bypass, LED or battery over a resistor is that the stage will have higher gain.
All the method sound better then cathode resistor itself I'm still impressed with the sound LED biased cathodes or RC bypass but none have come close to the difference of putting a battery for cathode bias. Battery bias kept the dramatic improvement in the bass and improve the presentation in the mid and upper range.
BTW, very nice shot I hope, u get the adapters working soon
)) and do some picture, ceramic base and teflon socket it may look cool!