Measured small E14 core isolation 1:1 600:600 transfomers

I've bought 10 of these for next to nothing from ebay. I needed cheap ground loop killers for my HT 7.2 setup using KRK monitors.
I've measured some other transformers I had at home for subs (I'll connect my diy subs into the whole mess) - strange and interesting results.

I have yet to install these, they seem fine for use, as it's just HT, so .. -3db is at 35 Hz, but these monitors hardly go to 50/60 hz anyway and they are at around -1.5 db there, so who gives a shit. Only issue I see is the relatively large 3rd harmonic, especially in bass area. Don't know still if that's going to be audible / too distracting.

I've read somewhere about a cool little optically based audio isolator (in analog domain, no adc-dac conversion) similar to optocouplers, but works for sinusoidal DC apparently with almost no distortion and great linearity, chip in question is il300.
This seemed much simpler to do. il300 would need some more electronics and work, and I don't really have the patience for that. I'll find it if these prove to be shit anyway.

Measured on Steinberg UR22 with REW, calibrated the card.

Obviously a proper isolation transformer is the way to go with all the complex design, internal Faraday cages and what nots...but I just don't want to buy 9 Lundahl-s for something like a HT setup I'll use once a week.