Sorry for the delay in getting back to you on this immattewj. Too much going on in my life lately. Getting ready for a knee replacement soon also.
Just because you’ve pretty much matched the channels voltage wise, doesn’t necessarily mean you’re gonna get perfect dynamics since you have such a difference in gain of the first tubes on balanced input tubes. If there’s a big difference here on the input and then you’re trying to make up the gain afterward, in the line section it could change the sound in ways of the dynamics or image.
Also the important thing is to have equal output on all four phases of the signal. Meaning the left positive and negative phases and right positive and negative phases need to all be equal.
on your question as far as opening up the bottom, yes, that has to be done and they can be turned upside down on top of some foam or rubber. The tubes don’t get super hot so we never really had a problem with it, melting, some good packing foam. No cage is needed.
In a home environment you can simply do what you’re doing using a test signal in to the preamp and using a voltmeter to measure the AC output with the volume all the way up.
You should always start with the preamp set to a single ended input. Your DAC or CD players usually outputs about 2V into the preamp.
Now with signal into the RCA input you measure the RCA single ended output. Step 1: Now swap tubes 1 and 2 with other tubes until you have equal output at the R&L RCA outputs.
Step 2: Continue with the signal into the RCA single ended inputs. Now with your AC voltmeter measure the balanced outputs. Pin two of the XLR is the positive phase and pin three is the negative phase. If these are not equal the potentiometer inside needs to be adjusted until the rear tube (neg phase) matches the front tube.
Tube 1 is the left positive phase tube, tube 4 is the negative phase.
tube 2 is the right positive phase, tube 5 is the negative phase.
The potentiometer inside, adjusts tube 4 and the other one adjusts tube 5 gain. The potentials are aligned with the channels.
Once you have all four phases equal on the XLR output jacks using the single input then you’re ready to move onto the balance input tubes.
*Do not touch these four tubes now. Move on to the balanced input tubes.
Now switch the selector switch to a balanced input and plug your DAC into that input. While playing a test on measure all four phases of the output at the XLR jacks. All four phases should be equal again, if not swap tubes 3 & 6 with each other or with other tubes until you match the channels.
Tubes 7 and 8 are just for the headphones. If you are not using headphones, you can leave these two tubes out. That will actually provide less current draw on the regulators for the filament supply.
For people who have an SLP-05 and are only using it single ended, you’re missing out and you are only using tubes 1 and 2. Nothing else. But you do have to have four tubes in the preamp, them being 1, 2, 4 & 5. Tubes 3 & 6 can be left out if you’re only using RCA single ended in and single ended out. if you leave tubes 4 and 5 out, you will not get any filament supply to tubes 1 and 2 as it uses a 12V supply with tubes in series.
Hope this helps! Let me know if you have any other questions.