$8k for speakers - new or used

Adding subs, one or two?  What about placement. REL or JL Audio?

Definitely two.  I have successfully put subs between main speakers and rack, minimum 12 inches from back wall.  

Of the (4) different sub brands I have owned JL Audio has the most punch with the deepest notes, the E110 or 12 are great.   

REL adds punch alongside your speakers but lacks the deepest notes.  
SVS is the best value, the SB3000 is excellent of you have the room.  

I would look at Aerial Acoustics line of speakers. Their sound signature is similar to B&W but with less of an upper midrange edge. Extremely dynamic speakers with great bass.

Give them a try!

@secretguy

I own and enjoy my two pair of Magnepans, but "punch" and "bass" aren’t among their notable characteristics LOL

@wspohn 

Yep, slam and bass. Not my favorite speakers for under $8k, but the fill the OP's demands for sure.