lewm,
I believe the dealer had it between the linestage and the amplifier. He had a very young tech who built the comparator box and would also make the mono adapters for customers (put them in various vintage component cases with RCA jacks). I don't recall what specific transformers were used, but some were current production and others were vintage; the most expensive being Western Electric transformers. Based on recommendations he found on the internet, he built one such mono adaptors using two transformers (both Western Electric transformers) and that was the best sounding, but a bit pricey.
I have no idea why transformers with zero gain sound better than a Y connector, although I sort of have a theory. I suspect that the transformers introduce some phase shifting that actually improves the sound.