SVS is primarily sold directly from the manufacturer. They can easily be adjusted and or positioned to sound less desirable than a REL.
The REL is a -6dB sub-bass speaker equipped with only basic crossover, 2 position phase switch and volume. Because its low frequency roll off is so dramatic it simply does't excite a rooms standing wave modes and can be located most anywhere and often next to the speakers.
The SVS is an actual -3dB DSP equipped subwoofer. If its not located in the rooms standing wave mode it can over excite the rooms modes and possibly sound boomy. If the SVS is demonstrated in line with the speaker and the REL its likely out of its optimum position in a null. DSP is not intended as panacea for null or poor positioning rather to better match a speakers low frequency presentation.
If this looks to be the case and you find the REL to be more appealing to you, I'd suggest a dealer is pissing on your shoes.