Hi Changeout,
First off my room is larger than yours, 20 x 28x 10' ceiling opening to an adjacent 9x12x10, essentially an L shaped space, excellent accoustically. Low powered tube amps work well with certain types of music and in smaller rooms. The DeCapo's as well as the Reflectors are reasonably efficient and easy to drive with lower powered tube amps. SET amps? depending on musical tastes and room size. My taste in music runs the full gamut from full orchestral to Rock to intimate jazz, Brazilian and latin music of all variety and vocal from solo to choral. For this music to be played with realism IME requires POWER. I am using a Quicksilver Silver 88 amp with KT150 tubes, very low distortion, low output impedance and without any question among the very finest, nuanced amplifiers I've persopnlly experienced including numerous SET amps of many varieties.
With you room size and possiby your musical taste one of your stable of low powered tube amps might work just fine with either of the Reference 3a speakers. In any case regardless of where you ultimately end up, enjoy your journey! The fun part of this hobby is the search for and anticipation of raising our musical enjoyment. Of the speakers mentioned in this thread, I have found the powered ATCs to be intriguing, love to hear them.