Improving drivability of driver

[ackcheng]

Got interesting in improving the damping factor for OTL from transcendent OTL T8

Here is the notes form Transcendent site
"Let’s look at the Transcendent T8 as an example. It uses 8, EL509 tubes in push-pull. Each 509 has a plate impedance of about 150 ohms. 150 ÷8 = 18.75 ohms. This is unacceptable. Negative feedback is employed to achieve proper performance. The amplifier uses 33 dB of negative feedback which reduces the output impedance to 0.4 ohm thereby achieving outstanding woofer control and the ability to drive 4 ohm speakers. This specification was verified by Stereophile when they reviewed the amplifier."

Actually, I don't know how is 0.4ohms derived but guess there are 2 ways we can improve the drivability of the drivers
1. Use more output tubes
2. Connecting 2 drivers in series and increase the impedience

Am I correct? Can someoe kindly tell me how is 0.4ohm derived?