I’ve got the circuit for the Yves Cochet PP Un head amp somewhere. At the time I worked on it it belonged to Jake Mortlock but I understand @pmac bought it shortly before relocating and I’m not sure if it’s seen the light of day with him yet. It’s got slightly more gain than the Leach one (30dB I think) and a 20ohm Zin, although you could trivially lower that with a load resistor. The only reason I’m mentioning it is that I think Jake said he’d tried it with an Io and it had worked.
Active circuitry running at the microvolt levels out of an Io needs to be designed and built in such a way as to minimise noise. Leach’s use of a battery can be a big help with that. The PP Un had a mains supply and the bulk of its noise (not that there was much) was at 50Hz.
The circuit was more complex than the Leach one, but not ridiculously so. It can’t have been that tricky given that I was able to repair it !
Leach’s notes said that the transistors were selected for their super low noise, and were actually 4db better s/n ratio than the ultra low noise audio ones he’d originally selected. That coupled with a battery might work, I’ve really no idea.
Low transistor noise is important. But it’s not the only type of noise. One way and another they all get on your nerves after a bit (including hum and hash pickup in SUTs). The trick is to get them comfortably below the surface noise on the record.