Wow - this is getting more interesting, and more complicated, by the minute! An interesting historical puzzle for sure! Yes, the TFA-204 is still one of the best available. I assume you are getting the TFA-204 series feed transformer, not the TFA-2004 parallel feed version.
I now have the following references:
1) My own original SEX manual, version 1.1, 1996.
2) My article "Braniac's S.E.X. Changes, VALVE v.4 no.10 October 1997 pp. 15-16
3) The circuit "ActiveS.E.X. from VALVE v.6 no. 1 1999 p. 13 which depicts an amp converted to parallel feed and with a C4S plate load on the driver, and also incorporating some of the ideas from 2)
The circuit 3) is the most recent, however it does not include a power supply circuit, and it depicts the parallel feed mod (called ParaS.E.X.) which is different from the TFA-204 mod. Here are some notes to make sense of all this:
Power supply: The original in 1) was a CLC (10uF/10henries/10uF) followed by a series 1K power resistor. The modification described in 3) is to replace the last 10uF with 47uF450v and remove the 1K resistor (replace it with wire). This will give about 410v at the output plates.
The same circuit 3) shows a 6Kohm, 3Watt resistor and a second 47uF 450v capacitor; this is a decoupling network for the driver stage, which drops the voltage to a safe level (300v) for the C4S transistor. Notice this - there are TWO 47uF/450v capacitors per channel.
The power supply in 1) also shows a voltage divider of 270K and 100K, with a 0.1uF capacitor across the 100K. The hot end of the 270K goes to the power supply 410v, the common point where the two resistors meet goes to pin 8 of the tubes, and the other end of the 100K goes to ground. I would suggest reducing teh 100K to around 50K; the higher value was only needed with the original driver circuit. You can put a second 100K in parallel with the existing one to get 50K.
Finally, the 6.3v winding goes to pin B7 and B8 with a twisted pair. Another twisted pair connects B7 to A7, and B8 to A8.
Output stage: This is pins 1, 2, and 3 of the tubes. They are in parallel - A1 is connected to B1, A2 to B2, and A3 to B3. In the circuit 3) the cathodes (A3/B3) are connected to the old output transformer but for the TFA-204 they should connect to a 390 ohm resistor whose other end is grounded. The circuit 3) shows that to be a 390 ohm 2 watt resistor but for the TFA-204 I recommend a 5 watt resistor. This resistor is bypassed with a 220uF 35v electrolytic, as shown in 3).
The grids (A1/B1) goes to ground through a 270K resistor, and to the driver through a 0.47uF capacitor. I recommend a small grid stopper, 200 to 500 ohms metal film, connected right at each socket pin 1, with the other wires to pin 1 going to the other end of the stopper resistor.
The plates (A2/B2) are shown in 3) going to the old output transformer and a parafeed capacitor. For the TFA-204, the plates should go to the blue output transformer wire. The red output transformer wire goes to the power supply 410v point.
Driver stage: The driver triodes are pins 4, 5, and 6. They are in parallel, same as the power section A4 connected to B4, A5 to B5, and A6 to B6. The grid (A4/B4) goes to the input RCA jack as usual.
The cathode (A6/B6) goes to ground through a 500 ohm resistor. Circuit 3) shows 1000 ohms, which is incorrect. The 500 ohms resistor must be rated at least 1/2 watt; I would use a 1 watt myself. It can be any value from 470 to 500 ohms. It is bypassed with a 220uF or greater capacitor rated 16 to 35 volts.
The C4S input comes from the second 47uF capacitor, about 300volts. The C4S output goes to the plate (A5/B5) and from there to the 0.47uF coupling capacitor. The C4S has a ground lead to signal ground as well. Note that the C4S has three 50K 1W resistors in series from the LED string to ground; this is an unusual arrangement so be sure you have it correct.