I'll just share my setup where it is currently being done by phases. As of now, my craving of adding a sub has been minimized after doing the following:
1. Upgrading the front speakers to a set of speaker separates that is known to have a good midbass response(ie. Morel Tempo 6, DLS RM6.2/UP6i, Focal PS165FX etc).
2. Install a warm sounding amp(Audison, Genesis, DLS etc.)
3. Get deadening right(multiple layers and cover the holes)
4. Add a processor(beef up the sinewave on the lower end)

Really happy with the outcome. A sub will boost the lower end presence, but I am satisfied with the bass I am hearing right now on this space saving setup.