I wanted to post this to both give other owners who may have zbox hum problems a possible solution and at the same time invite comments as to the draw backs of doing this mod.  So basically on low powered amps (4WPC or less) the Zbox I have worked acceptable.  As I put the zbox on other amps, hum/buzz increased as the gain or power of the amp increased.  Meaning, the box was quiet on a 4WPC amp, but got noisy on a 30WPC amp, and made more noise on a 50WPC amp, 130WPC amp, 160wpc amp, 250WPC amp, and 380WPC amp.  Didn't matter of SS or tube .  I poked around the internet and found a few folks with the same problem and some of them did a photo shoot that involved re-bundling wires and other mods.  None of which worked.  Ultimately someone on the DIYAUDIO forum had me check pin 9 to see if it was grounded.  Pin 9 is center tap for the filiments which were powered by AC in the zbox.  Grounding the center tap was proposed to provide the AC heater circut a DC ground reference.  Key point was to make sure there was no other grounds at play in the AC heater circut of the zBox.  I checked the circut end to end and confirmed that the circut had a totally floating AC circut with no grounds.  I connected pin 9 to ground and was amazed how quiet the box became.  Dead quiet no matter what amp I used and no matter what the sound level was.  I sent a few emails out to get feed back as to why this mod should not be done, and thus far, did not get anything back.  Not sure if this mod would work if you already tried the mod to use two 100ohm resisters, one on each leg of the heater circut.  I stayed away from the two 100 ohm resister mod becuase the current draw of that mod would be .1 of an amp and it looks like the zBox heater circut only has .3 of an amp available.  so that was a no go.  The grounding of pin 9 did not increase current requirements at all.  Make sure you ground pin 9 and not pin 1.  If in doubt, test the pin with a voltmeter and make sure you do not get an voltage.  If you get a voltage reading, your on pin 1 and not pin 9.  Proceed with cuation as there are high voltages in the zBox.  If you do not have the skills and training, do not attempt.

Definitely some fidelity changes since the noise floor is lowered.  Overall, the zBox does what it claims and musical details are more forward with the box than with out.  You get a perception of better lower mid range....but that might be due to the size of the input coupling capacitor, in the same sense a speaker sounds clearer if you use a sub with a high pass filter going to the main filters.  Leaving the zBox on all the time is probably the biggest feature.  The zBox draws very little power and I'm convinced leaving the zbox on all the time keeps the interconnects warmed up or charged.  Before solving the hum/buzz, I bought some OC2 tubes to do the power supply mod and have a few low current chokes for the power supply, but quite frankly, the ground on pin 9 did the trick so well that I'm not sure I'll do anything else with the box.

just purchased a second (used) zbox.  with a 50 WPC amp connected, found the same hum/buzz issue that existed on the first zbox (which I still have).  Opened up the second zbox and found a cleaner lay out, sheilded wires, max spacing between power supply and tube circuts.......everything looked good.  The second unit has a bronze volume knob and the fist unit has a chrome knob.  so I test pin 9 and it was not grounded.  checked continuity between pin 9 and pins 4/5 to make sure I properly identified pin 9 instead of pin 1.  so with pin 9 identified, I located the small hole at the face of the case box and scratched off some paint around the hole on the inside so that my ground terminal would have good contact.  soldered about 3 inches of wire to a eye terminal on one end and pin 9 on the other.  Used one of the access cover machine screws with a lock washer nut and passed the screw thru the case hole from the outside and bolted the eye terminal using the lock washer and nut.  tighten'd just enough to get good contact with out over tightening.  used an ohm meter across the case screw holes and pin 9 to confirm I had a good ground.  closed her up and tested.  the hum buzz was gone.  so the mod was needed on two units with sybstantially different build qualities and in both cases the mod mad a dramatic difference in reducing the back ground noise.  Again, if you do not know your way around with tube circuts and a soldering gun, and or have concerns about any remaining waranties, I would not recommend you go down this path.  this info is for the DIY'er who knows what they are doing.

Hi Speakerfritz!
I've been following your thread here and at DIYaudio. I just finished building a Zstage from scratch last week and I can't get rid of a hum (audible only if i place my ear a few inches from a 97db Hoyt-Bedford speakers). I have tried grounding pin 9, replacing the first filter cap (to 2200uf 200volts), twisted the heater wires, DC heater supply and removing the 15-ohm-series heater resistor to no avail.
Just this morning, I tried to remove the transformer and the hum was gone. I then transferred the tranny to the top of the chassis and I now have a super quiet preamp for the Taboo. I have a CSP2+ that I used with a Zen.

In this circuit, are pins 4 and five connected together?  Would someone please explain the STOCK, un-modified heater wiring in the Z- box?  I'mmassuming that pins 4/5 are tied together with pin 9 providing the other connection point for a parallel supply.  If only pin 9 is grounded, there may be mild imbalance between to two sections of the filament supply.  The 100 ohm resistors, which do not carry heavy current in this case, are thus connected between pins 4/5 to ground for one resistor, and from pin 9 to ground for the other. However, these might be wired in series, with pins 4 and 5 only used.  That's what I'm interested in learning because it determines how hum reduction is employed.