Hello Ray
I too have the acclaimed 2031A monitors with MOTU interface. Getting another pair of 2031A monitors provides flexibility in case you later decide that you'd like to incorporate 2 stereo pairs in front vs having a set dedicated for surround.
Which Presonus Eris models do they list on sale? If its the Eirs 3.5 set I wouldn't recommend them as they are a bit undersized for surround. Remember the pedal division has surround ranks too, so its not just ambient, reflective sound akin to a home theatre setup. The two surround sets I own give you a control panel with sliders (faders) to adjust the volume by organ division to the front and rear channels. For my setup, I simply lowered the volume for the pedal division in the rear speakers. I actually lowered it quite a bit even though they have 8" bass drivers and can handle some bass. During a MIDI playback when I first set them up, I noticed the bass drivers of the surround speakers were really oscillating hard for a sustained pedal note which prompted me to make the adjustment.
For my surround sound speakers, I'm using the Behringer 2031P (passive) models which are basically identical to the "A" models just no amplifier. I bought these used in mint condition and the price was too good to pass up plus I got an aesthetic bonus of having all matching speakers.
As of this writing, the Behringer 2031As are on back order from
all vendors. You'll have to find a used pair.
Finally if indeed you are only looking for some ambient surround ranks at minimal cost, there are 2 forum members that have given the Polk audio TSi100 and TSi200 speakers high marks for Hauptwerk surround sound implementation. I personally haven't heard them but plan to someday. You'll need to supply an amplifier for them.
https://tinyurl.com/yp92ykj7Danny B.