5751/12AT7 preamp tubes in Diezel Herbert

I have bought some 12AT7 and 5751 preamp tubes for gain/noise reduction experiment.
They are Chinese Shuguang 12AT7, Russian Electro-Harmonix 12AT7EH and Sovtek 5751.

=Gain Factor=
12AX7/7025/ECC83  is 100mu
5751                           is   70mu (30% gain less than 12AX7)
12AT7/ECC81           is   60mu (40% gain less than 12AX7)
12AU7/ECC82           is   20mu
You can ONLY put 12AT7 or 5751 in Diezel for gain reducing,
NOT any others like 12AY7, 12AU7 or 12AV7.



=Preamp Tube Layouts=
All the stock preamp tubes in Herbert are Ruby selected 12AX7HG higain tubes (Chinese made).
Herbert:
V1 Used by all channels (nearest the input jack)
V2 Used by Ch3
V3 Used by all channels
V4 Mixer & Loop Driver
V5 Return Driver
V6 Phase Inverter

Firstly, I put Chinese 12AT7 on V2.
Comparing with Ch2(+ mode), the gain and hiss/hum noise of Ch3 are obviously reduced.
Gain level in Ch3 is still much more than Ch2 even changed with 12AT7.
Tone and compression wise, they are still keep the same before and after.

After that, I changed 12AX7 on V3 with another 12AT7, Electro-Harmonix 12AT7EH.
Both Ch2 and Ch3 are quiet down. Just a little hiss noise on maximum gain setting.
Still NO improvement in compression for both channels.
The volume of Ch1 is dropped, so I need to crank up the channel volume higher.

Lastly, I put Sovtek 5751 in V1 position. Now, all three preamp tubes in my Herbert are lower gain tubes.
Ch2 is tame now, not good for metal anymore but great for blues and rock.
Ch3 is still highly compressed as usual.

I did change the V6 Phase-Inverter to Electro-Harmonix 12AT7EH.
There's NO any difference in both dynamics and tone.

=Summary=
Lower gain factor preamp tubes like 12AT7 and 5751 reduce gain and noise obviously,
but they didn't open up the compression feeling from Herbert. And I still love the Ruby selected
12AX7HG that came with stock, they are great with Herbert.