Here are some of my thoughts about SL and subs:
– Generally SLs do not need subs! It is perfect enough design that can cover entire frequencies spectrum
– Do subs help ? Yes! But primary not in bass like many people think. They help in 2 important things :
1. Soundstage
2. Instruments body and texture
– Question is where to put crossover point. SL have one of the best midranges on the market, matched only by Quad 57. If you go up with crossover point you will ruin lower/upper bass and midrange as well. My crossover point is 25Hz. Now you can ask why that so low point has anything with midrange – it does ! Huge! The key is overtones from mids that come very low. You can not hear them individually but in the context of music as a whole – they have big influence
I have pair of REL 212/SX and with their design they match flawlessly with my SL U-745
D