I initially set my 4 subs up using two Y splitters directly from my preamp. After reading about possible impedance issues and at the suggestion of others, I purchased a used iFi Micro iTube2.
I now run the left and right output from my preamp into the iTube2 and have the Y splitters coming out of the output of the iTube 2. Two of the subs are connected by RCA cables and two are connected wirelessly. I didn't hear any noticeable difference after inserting the iTube 2, you might be fine running Y splitters right out of the preamp.
I wish I could remember where I read that information as it was explained better than I can why impedance matching with multiple subs could be a problem. It might have been one of these:
https://forum.audiogon.com/discussions/preamp-connection-to-multiple-powered-subwoofers
https://forum.audiogon.com/discussions/possible-impedance-matching-issue-with-subwoofer
It was relatively inexpensive to add the iTube 2 and I decided "better safe than sorry".
This approach allowed me to make individual adjustments to each sub, which was important in my case due to their locations. For example, I have the level down on the one in front which is pretty close to my turntable as well as one that is relatively close to my listening position. I have the crossover levels higher on the front subs than the ones in back.
Hopefully someone with better technical knowledge can speak to the issue (or perhaps non-issue) of impedance matching your preamp to multiple subs and the use of a Y splitter.