Size matters when it comes to bass trapping. I find it difficult to believe that a couple of throw pillows in the tricorners can make an impact. Even purpose-built traps which are significantly larger and designed for that purpose are of limited efficacy below 100 Hz unless you opt for the limited bandwidth version. The picture below shows the performance of the GIK TriTrap, dimensions and construction of which are much different from a throw pillow.
I employ Vicoustics bass traps in the front and GIK traps in the back of the room.
Many would say this is appropriate and adequate treatment, but it really only begins to treat the issues. The curves below show the effects of room treatment alone (blue) versus multiple subwoofers and room treatment (green) in terms of both frequency extension but perhaps more importantly, in terms of a smoother frequency response with less dramatic peaks and nulls.
In my opinion, it is not the frequency extension that is the main benefit of multiple subs because one can go low with just one sub. It is the smoother response and diminished modal aberrations (ringing) that are most important as demonstrated by the waterfalls below.
Throwing a couple of small subs in your room will have a much greater benefit than a couple of pillows. I appreciate there is a significant dollar difference, but it is money better spent than on new amplification, source, or possibly even speakers. In terms of WAF, discreet sub placement may be less likely to attract attention than pillows in the upper corners of the room.