Tube mic quieter in Fig-8?

The noise floor in figure 8 can be also lower because of the side signal rejection.
In full fig 8 you completely eliminate side signal and depending on the recording place this can be a big and by chance the biggest factor in the difference of noisefloor behaviour.
I would look into this first and see if there is something to improve for the cardiodid setting in your place. (room acoustics)

As for the different sides of the capsule, you can check that e.g. if you record a signal in fig8, then turn the mic 180 degree and record again. Most probably one of the recordings sound better or louder. This is the side you want to have frontwise in cardiodid. I guess this is the easiest way to find out if it makes sense to open the mic and flip the capsule.

Hm, i am not sure about if this topic might fit better in the Pro Audio forum section, since it's more about mic behaviour than actual modification or diy electronics, Johnkenn ?

Self-noise of a mic really is an important measure. So in the end all depends how much noise there is practically in your recordings. Best way to figure this out is to leave the mic in the booth and just see how much noise you actually get in your DAW.
The specs of mics usually specify A-weighted self-noise, so the spectral behavior of the noise matters. A-weighted means it is taken into account that psychoacoustically we percieve basses in noise less much less than higher frequencies. This said, there are some guidelines what is acceptable noise.
The quietest mics there are can reach below 10dB self-noise, which is better than what almost every recording room adds to the noisefloor.
Most people find everything below 20dBA totally sufficient for everything except for recording of very quiet sources. Above that the noise becomes noticable, definitely. More than 24dBA is unacceptable except on very loud sources and normally does not qialify for studio works.
Signal-to-noise levels are measured by standard with a sound pressure level of 94dB/1 pascal, so the calculation is
Signal-to-noise level A-weighted (dBA) = 94dB - Self-Noise A-weighted(dBA)
However, what you see in your DAW's VUs normally is unweighted, so it is definitely higher.
This all said, what matters for you is the *actual* noise you get in your track.
So, to get a reality based figure, set your levels to get an RMS recording level of -18dB shown in your DAW, loudest peaks shall not go hotter than -6dB at any time. This way you get nice dynamic recordings.
Now see, what actual noisefloor you get if you let the mic alone and verify subjectively, how obstrusive you percieve the noise if you set the playback level of your monitoring at a comfortable level you normally use.
If you are picky, you can start a frequency/noise measurement sweep in the recording booth to see the spectral behaviour of the noise with one of the PC-based analyzing programs, don't know if there are similar programs for Mac...
But your DAW software most probably has possibilities to show up the spectrum of the noise you get. And from this you can come closer to the sources of the noise and if there is something obviously wrong and can be improved. You may also see hum etc.. To get a good picture of the noise content, just record 30-60 seconds of the mic alone in the booth, normalize this and watch in the analyzer of your DAW. Now you can see which frequencies are critical, or if there is more like white noise (all over the place) and so on.
After setting up the conservative level as i wrote before, how much noise and hum shows up in the meter anyway?
less than -75 dB? I would not bother at all. At least if you don't record 24 channels thru the tube mic in your mix (noise DOES add up with track count). I would be perfectly happy when achieving -73...-72 or even -70 dB on my DAWs scale across the audio band (22khz) this way, maybe even more noise depending how the spectral behaviour is and first - how *perceivable* it really is. Your ear IS a very good measurement instrument to decide how acceptable the noise is. There can be very annoying noise or maybe even a smooth warm unobstrusive floor, you can not see that with a VU meter.
Also, use a clean pre with the tube mic if noise bothers you. As clean and quiet as possible. Make sure you do not hear the pre noise and blame the mic if your pre is noisy at the maybe 30dB gain you might only need. Use good mic cable. Sometimes a lower impedance input on the micpre which might have to be compensated with more gain does the trick.

Wow, now i wrote much more than i intended to. I hope you can make use of it to check if you can live with the noise of your mic. It does matter that you can go to record beeing relaxed and not beeing unsure about the noisefloor...

First off, thank you both for your input.

Smallbutfine: I appreciate the lesson. I'm happy to learn more about ways to analyze my gear's performance and hopefully how to optimize my work product. I tried what you suggested about turning the mic around in Fig-8 and discovered that backside of the capsule actually (and not shockingly) sounds like crap compared to the front so I'll definitely be replacing that sooner than later, but after I give it a noise floor check as you described for better comparison.

Ward: Thank you to you, too. I'm again relieved to hear that this particular issue is a non-issue.

For now, I'll consider the issue resolved. Thanks again, Dudes of RGO! We now return you to your current programming...

Thanks Dave!
Yeah, I'm definitely going to replace those two Zeners in the PS with 1n4759's. I can probably rule out the tube since I've tried 3 different tubes: AY7, AX7 and AU7 and they all produce pretty much the same noise floor, the AX7 being a bit more noisy. Like you said, I'm fairly certain the capsule is the main culprit though. I'll give it some thought. I may just stick with some kind of 67/87 style cap though since that's the tonality that seems to work best for what I do and the sound my clients want from me.

Thanks again, Dave. Appreciate the input.

That sounds pretty good. I'm in no rush so I'm going to give it some thought as to what I feel like ultimately accomplishing with this mic. I only do VO so my needs are pretty limited but its worth considering if I do want to go with another tonality just for grins and giggles. I'm sure I'll be in touch at some point!

Yeah I would say the perceived self noise is often lower in figure 8 emphasis on perceived!

I've heard you can remove noise in a tube by burning them in at higher voltages.  It is suppose burn the crud off the tubes.  I have heard it can work but can also kill the tubes.  So it is a gamble.  But if the tube is noise and useless then I guess it is worth it.

I haven't tried it yet so I don't really know how it works or doesn't work.