To continue my trial for tone modifications using lower-gain tubes, I tested 6 old production 12AT7 tubes from Ebay. The are probably used but test from good to strong, with 2 RCAs in original box. Not surprisingly, I like 12AT7 (gain =60) in general over 5751 (gain=70) in the 12AX7 (gain=100) position .
1 Telefunken (made in Germany): very good presence in midrange and bass, but a bit thin in the treble
2. Mullard (made in Britain): sweet midrange, but a bit subdued in the treble
3. RCA 12AZ7 (1955 made in USA): very low noise, very balanced across all frequencies, good resolution. My second favorite of the bunch.
4. RCA12AZA (1960 made in USA): warm and powerful sound, treble is a bit subdued, very similar sound to Philips ECG JAN 5751
5. Another Telefunken, high noise in the right channel.
6. Ameperex Bugle Boy, smeared branding. This one is the best. The tube has a flap foil attached to its "D" getter, a very distinct feature that allowed me to Google its origin back to Holland in the 50s and 60s.
To date, I have tried 19 different tubes in the 12AX7 position. Overall, best results are achieved with tubes of lower gain factors. As for individual tubes, everyone of them sounds different. Those that sound sweet in the midrange or strong have thin or subdued in the treble. Those sound super-detailed overemphasize the treble. So in my experience, unusually sweet sound, strong bass and surprising details all come at the price of some spectrum unbalance. Depending on the music and listening preferences, the trade-off can be good. But I want the faithful reproduction of acoustic violins, so I prefer more neutral presentations.
I also use sensitive headphones to evaluate the noise level, and in general low noise tubes give better clarity and resolution. In the end I prefer low-noise tubes with are even across different frequencies, the neutral, clean-sounding tubes at all positions in Cayin HA-1A.
The only surprise came with the Amperex 12AT7. At first it seems that it could enhance the ambiance, the spatial resolution, the warmth, and the clarity simultaneously. Yet it's noise level is not especially low still behaves balanced across different frequencies. I have tried thirty 12A*7 compatible tubes in the Cayin, and this is the only tube that gives surprising results. It is a mystery to me.