Did you keep the knobs the same when plugging in the subwoofer?
Wayback I contacted Edifier support to ask about the sub out and if it cut the bass respnse from the speakers and sent to the sub specifically - this is the reply I received:
Thanks for reaching out. The “Sub Out” port on the rear panel of the active speaker has a detection function.
When the port is connected with an external subwoofer, the speaker’s own bass will be reduced, and the bass will be mainly output from the subwoofer connected(frequencies of 105Hz and below will be cut from the speakers letting the subwoofer take over the bass effects).
Now, in reality I did not find this to be all that accurate. there was some reduction but it seemed to be on volume level as a whole not a frequency shift towards the sub for bass. I ended up just using them without sub. Still have them to this day on a desk and another set on my game desk. They are very nice all on their own even w/o sub. And FYI I leave treble and bass both in the center default position. Works well in my rooms - YMMV. I think you are going to have to experiment to find what works best for your room area and conditions.
Also, for my primary listening desk I finally opted for a set passive speakers, amp with sw/out, and a sub. It was just easier to do in the long run. Nothing special as I listen to headphones more there ,and my investment is in that source chain.
My speaker setup is nothing special, but gets the job done and came in not much more than a set of the 1700BTs + sub if I recall correctly.
SMSL SA300 (not using the built in DAC, I’m sending it a signal from my main DAC)
Elac 1010 sub