Should I: A. Order a 5-headed hydra for my 5 IEC system
or B. Order a 4-headed hydra and keep using my Powerline on the 555DR or SC2?
The risk is that losing the Powerline on one of the power supplies would drag down SQ more than having a Hydra improves SQ.
The problem with ordering a 5-headed Hydra is that if the Powerline is better than a Hydra head, the remaining unused head becomes an extra aerial and injects noise into the system.
(Option A means that one Hydra plug is plugged into the double socket behind the hifi - Option B means that there’s a Powerline and a 4-headed Hydra plugged into those 2 sockets).
I had a similar quandry a while ago and decided to go down the ‘smaller hydra with separate cable to other socket behind stereo’ route. I think a lot of it comes down to earthing - a common/star earth matters more to some things than others! I was happy with the result and you could always resell your hydra and get a 5 head one in the worst case - they command a respectable price on eBay etc.
Just as an aside - that’s a really interesting system you have - real focus on power supply quality. I’m sure someone once said ‘the power supply IS the amplifier’ - might well be true!
I used a 5 headed Hydra. Then I started buying Powerlines. As I added Powerlines the number of redundant Hydra heads increased. At no stage did the addition of a Powerline prove to be anything other than beneficial. I wouldn’t worry about the theoretical problem of redundant heads or ‘discontinuities’ in star earthing.
It was a hunch based on already having one very decent power cable and, of course, because both are being plugged into the same 2 gang socket, they share an earth anyway.