Have your friend email me, I have a neat and useful improvement to the circuit. One of my mates up here has built a pair and reported favourably on the tweak. It is also free.

For manualblock, the Seth is a simple PP 2A3 circuit. Peerless S-240-Q output, phase splitting choke, and a high gain triode driving one end of it. CCS or resistive parafeed drive to the choke which is attached to the 2A3's grids. One of Magnaquest's distributors dreamed it up. It became viable with the modification to Mike's EXO-173 he and I worked out. I published a frequency response table on the prototype( on AA's Tube DIY ). Since I like doing the phase splitting with a tube( tubes actually ), I was never happy with driving an interleaved CT choke with a balanced drive. Some experimentation I did indicated it was likely due to the added winding capacitance the interleave caused.

For a 2A3 driving a less-than-flat speaker impedance, 6k6 is a better a-a load. If you want to stay in the Peerless 20-20 series, the OPTx in question goes by S-230-Q. This is the OPTx I am going to get wound for my wife's 2A3 project, Galahad. The Peerless spec on idle current is 70 mA/leg which is well within the 2A3's 60-65 mA idle current.
cheers,
Douglas