Does using a powered subwoofer make better sound?

I have Vienna Acoustic Mozarts. Wonderful speakers but their one weakness is below 40 Hz. I don't want to "harm" the beautiful mid-range and highs they produce. Would a powered sub give me the extra kick in bass I'd like? If so which one to buy?

System Theta Miles / Pro Prime IIIa DAC
Audio Research LS 15 Preamp
BAT VK-60 amp