If your budget will stretch to about $750 for a used sub, there are very few subs that are more musical than the Vandersteen 2Wq. However, Vandersteen recommends that this sub be used with a fairly full-range main speaker, i.e., one that has pretty flat response down to about 40 Hz. I think your Studio 100's meet that criteria, but you might want to check.
The 2Wq sub(s) utilize a passive crossover between the preamp and the power amp which allows impedance matching with your power amp, and the result is virtually seamless blending between the main speakers and the sub. The 2Wq has a dial that allows you to adjust the "Q" (amount of damping), and because the 2Wq is a sealed design, you get very tight, clean, musical bass.
The 2Wq sub(s) utilize a passive crossover between the preamp and the power amp which allows impedance matching with your power amp, and the result is virtually seamless blending between the main speakers and the sub. The 2Wq has a dial that allows you to adjust the "Q" (amount of damping), and because the 2Wq is a sealed design, you get very tight, clean, musical bass.