12BZ7 ///12AX7

I'm not sure how the extra current capability of the 12bz7 over 12ax7 would be an advantage? Most designers place 12ax7 for their gain, not current. They're never reasonably going to be asked to drive a hard load, so you're perhaps not gaining anything meaningful. However the 12au7 is often used as a driver or follower, and I've always heard improvements by subbing in the much stouter 12bh7.

Yes, you have to be careful that you have enough overhead for the extra heater current draw. Always consider the component. For a preamp with 6x 12au7, yeah you're probably in trouble subbing 12bh7's and doubling the heater requirements. For my Rogue Apollos with 4x 12au7 and 12x KT120 - well more than 90% of the heater current is sucked down by the power tubes and an extra 0.3A per 12bh7 didn't pose a problem. And yes it sounded a lot better too!

@mozartfan,
I'm not understanding your opinion that the 12AU7/ECC82 sucks. In the Asylum shootout the clear winner was the Brimar Gold Aero Platinum grade 12AU7A.

There are superior drop in substitutes such as the 5814, 5814A, CV4003 Mullard & Brimar, 7316 & 7308 Amperex/Philips. All are in the AU7 family, all low-noise tubes. Many preamps and amps do not recommend using higher current tubes.

7308 is a 6dj8 equivalent, not 12au7.
The Defy 7 has the 12au7 as input tube, the 12ax7 is the driver tube. The extra current capability of a 12bh7 is unused in this circuit. Same for a 12bz7 in place of the 12ax7 at the same operating current. The miller capacitance is higher in the 12bz7 so the high frequency response will be more restricted. Without modifying the circuit, the performance overall should be worse.

Those spec sheets on the 
BH/BZ/AX, all look similiar.
But note:
I am never using the vol on the preamp with much gain, My room is 10X12/8 ft ceiling, why crank it up??
= No reason at all.
The Jadis's trans , inspite of RUMORS of a  *previous model* (a  rumor I ain;'t buying). , the Jadis builds like tanks, indestructible. 
If the BZ, BZ has over heating issues, the tube will blow before anything inisde the Jadis./

Besides there are resistors inside the Jadis, either the  resist or tube will blow,,,
I will carefully note the BZ/BH's temp, while employed.

Also I made some changes in my speaker wiring, Added silver to the copper.
Thing is, as i was making that mod,,i noted one biwire was crossed wrong. I had one neg going to the opposite post, That is the left channel had 
the black lines crisscrossed, and THIS was no doubt causing HAVOC/HIGH DISTORTION in that cahnnel and completely annihiliating /destroying, mulitiating the fq resolution. 
.
So correcting that ~~unforgivable~~ error  
OKUS adding the silver sterling 92% silver line with fat copper strand.
= the resolution of fq's in super.
This Jadis sings now.
When I put more vol on the Lite LS9  Jadis clone, there is no ~~complete breakdown~~~ of fq's = the high level of distortion (which i attributed to the pre's low level of build quality) has all disappeared.
Perhaps the added silver did nothing at all to gain high fq resolution..= IOW was the whole issue the crisscrossed speaker wires??
Or did thev added silver add a  clean, clear, nice gain in resolution???
I have no idea and will not make any experiements with 
all copper vs 
silver/copper.
Time constraints.
The Bz/s'Bh's , will arrive late this week, will report back on how that tube roll went off. 
Let me finsih by saying the sound is completely 
~Non Fatiguing~ = Zero fatigue Factor~~ (aka ZFF)
I loistened to Mahavishnu Orchestra, Live Central park, and wasblown away at what serenity overcame me , which never on previous systems experienced.
That record is not high fidelity studio, Diana Krall was mesmerizing. 
I could retire from this mod adventure with what i am getting from the system. 
But as a  seeker of high fidelity, this desire to make futher discoveries is calling me onwards.
I have 20 M SGO caps to be employed in my cayin cd 17 Mark1.
later this year will rebuild the Thors xovers with M high level caps. 
Along with the cayin SC6.
The SC6 employs 4 AU's,,well you already know what i plan to do, and will ck the E80CC's tempt while in play, and also the heat level on tops of trans. 
Cayin is like Jdais, build quality impeccable, ~~build like tanks~
YT vid update comming  Sunday. will post a  link.
But as a  DIY member noted, a  YT vid offers little in forminga  opinion on a stereo sound.
I agree in part only.
After listening to countless YT uploads of stereo's in performance, I can now  hear through the compressed replay.

Here is a  example froma  YT vid with a  guy showing off the M high level caps mod in his amp. 
I can hear the M caps singing GORGEOUS.

Not the vid I was looking for , yet another testimony to M caps singing ~~Like Angels~~~

https://www.youtube.com/watch?v=3vLTeIDuWmM


Here is the reference vid i was looking for.
https://www.youtube.com/watch?v=XfqWgHqiPwQ

Now listen to this Class D/Class F sound with Jadis; audition room at a  stereo show

This is not the vid I was looking for, but it will work, showing how a  speaker can drag down a  complete $100K Jadis set up, to ~~Class F ~~ sound.

https://www.youtube.com/watch?v=CIut7iVo7DM&t=291s


and note carefully in this vid, where you can see one of the best mid/tweets ever made.
The Seas Millenium!!!, UNREAL, either the xover design/parts or that mid range is driving the entire Jadis + Millenium down to grade
Class F. 
No I can not grant a  ~~Grade D~~ to this horrific atrocious sound level. 
This is from a  Munich 2019 show

https://www.youtube.com/watch?v=Hzbz4b09bH8

So agisnt the opinion of the DIY member, a  stereo 's quality of resolution can be noted, at least on most vids. If you have a  
1) trained ear/sensibility 
2) practice makes perfect
3) listening attentively
4)  replay as often as needed to draw some conclusions.
Every Jadis showroom upload on YT, , has these horrific speaker set ups, dragging, what otherwise could be a  Pure Class A sound down to
~~Pure Class D~~ And in this Munich show, down to  pure class F Minus resolution of fq's.