What you are suffering from is boundary cancellation. You have also got standing waves that are cancelling low frequencies...I do believe the Eris 8's are too much for that room, however the 5's would likely have issues as well...The Eris 8's are actually a bit bass heavy, so that should tell you something.
Bass traps would help, but if that room is THAT small you might want to try putting the speakers on the long access wall...One MAJOR problem I see is the speaker in the corner, this may sound odd considering your statements above... but corner loading will INCREASE bass output considerably... "Each boundary will increase the low end by 3dB, so in a corner like that the corner walls will add 6dB of low end, and if it was on the floor in the corner that would be 9dB more that 2 times the normal output, BUT the high end isn't coupled like the low end...
So, why then isn't there WAY TOO MUCH low end? That would be standing wave cancellation. If you do some research ( I have a TON of this stuff @ dbx in the forum section READ me header sections. I have JBL 4406's and I have them well away from all boundaries, this way my speakers are in "free space" and @ their most flat response, and thus more accurate. Your going to need LOTS of room treatement,, and you might even want to consider Eris 4.5's and maybe a sub???