I have used both B&Z booster & the SP01 on the QED.
Neither worked well with the Bluetooth wireless set ups that I was trying & with the Seinheisser RS-160's (the best RF wireless set up I tried) you had to set up as QED audio out >> Booster >> TX >>>> RX (built into headphones so only way to configure). There was little or no improvement to the RS-160's configured like this & it isn't the ideal set up as SteelPhase has also said previously in regards to the SP01.
Ideally for wireless it should be QED audio out >> TX >>>> RX >> Booster >> headphones/speaker. You really need a Bluetooth or RF system with seperate transmitter (TX) & reciever (RX) not built in RX.
Using with wired headphones & speakers I found that with the QED the B&Z introduced a lot of background hiss at all volume levels but did improve volume.
The SP01 improved volume without any background hiss & while maybe not as "loud" as the B&Z was much better. It worked well but ideally I wanted wireless so gave up on using a booster preferring the RS-160's.
As Pat says above I had to rewire my audio cable in order to get the QED audio in both L&R channels thru the SP01.
The RS-160's are on sale at Bing Lee at the moment for $199 which is cheap.
https://www.binglee.com.au/sennheis...MIzsD4y4fn3wIVAw4rCh1TxQKrEAQYAyABEgLJT_D_BwE
Depending on your own hearing you may find these still a little low on volume with the QED but I found them good but did use them close to full volume.
I think a booster is really only necessary on the QED if you have hearing problems or like a louder background threshold.
The other plus with a booster is having convenient volume control.