Ordered IFinity Wireless transmitter/receiver:
https://www.amazon.com/gp/product/B00SJ49ZJQ/ for HT surrounds.
Cheap price, cheap looking, tiny two plastic boxes that are not much bigger than the tiny SMPS that come with them. Also comes with two sets of RCA-terminated wires.
Hooked them up to a DAC and to the back of JBL 306P Mk2 speakers... Pressed play, and it worked! Nothing to fuss with, transmitter and receiver automatically discovered each other. Music is playing, and sounds just fine, especially for HT surrounds. Needs RCA to XLR converter to connect to 306P, but I had those on hand.
Some quick measurements before installing this in the HT. THD+N is OK: 1KHz sine wave:
View attachment 34982
Frequency response, good to about 20kHz:
View attachment 34985
If someone is interested in using this with a sub, here's the FR below 20Hz:
View attachment 34995
Cross-talk seems good, around -90dB:
View attachment 34988
Jitter is also not awful for side channel speakers:
View attachment 34992
I'll try to set this up later and do some listening