Regarding the difference between the China 12AX7s we use in production and the so-called C9 or 9th generation 12AX7, our position is that there is none other than the normal progression towards continuing improvements in quality which is always ongoing. Hence the tubes we use are the same as those designated by others as C9. None of the suppliers we use can confirm or verify that this is anything more that creative marketing on the part of some 3rd tier tube resellers.
Most likely any difference you may hear would be attributed to either normal performance variations from one production run to the next, or just the difference between a fresh tube and one with a bit of mileage on it.
I'm still prodding our suppliers to do their own research, and will post an update as soon as I get more information. Until then, our position that this claim lacks substantive documentation stands.
Based on the most recent production batches of preamp tubes we evaluated, we will continue to ship our amplifiers with the same tube complement that we have been using all along:
V1 stage: 12AX7WB or V1 grade Selected Shuguang 12AX7A
All other 12AX7 positions: Low Noise Selected Shuguang 12AX7A
All 12AU7 or 12AT7 applications: Shuguang 12AU7 or 12AT7
Valvulator 1: 12AX7WB or JJ12AX7/ECC83
GP3 V1 and V2: 12AX7WB.
GP3 All other 12AX7 positions: Low Noise Selected Shuguang 12AX7A
Power amps: Low Noise Selected Shuguang 12AX7A and 12AU7.
Recently evaluated tubes:
Well balanced tonal spectrum, low noise with moderate microphonic tendencies, easily selected out in production. Excellent for clean and overdrive sounds.
Lower gain, somewhat less bass response, and not as bright as Shuguang, but not dull sounding. Very low microphonic tendencies.
Very good gain and low microphonics. Nice low end. Somewhat forward in the upper mids. Basically a fatter higher gain EH 12AX7. Appears to be an EH with a different getter.
Very good gain and low microphonics. Less low end than Tung-Sol and not as smooth on top. More pronounced in the upper mids.
Similar to the EH but generally a little less of everything. Nice balance and a good alternative to the Shuguang if you like less top and a little more mid.
Works well in Valvulator 1, which was surprising, since it did not fare as well in prior testing. Apparently the hum and microphonic issues have been addressed. A little darker sounding than the WB.
When used in the amplifier V1 stage, this tube performs similarly to the WB, but the mids are more pronounced, the top end is dullish and overall somewhat harsh sounding for high gain settings.
Hope you find this useful...