$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.

I would suggest Wilson Audio Watt/Puppy 8’s to pair with your ML No.333. I have ML No. 436 Monoblock’s powering a pair of Watt/Puppy 8’s which I find to be an excellent pairing being neither fatiguing nor forward.

One area of Interest to myself is which floor structure the D3’s are seated on and if the Speakers Sub-Plinth and Supplied Feet is the only mount in use being in direct contact with the Floor.

Additionally, my experience had with Diamond D4's did require the that the owner was informed there was a improvement to be found if the Toe was adjusted. Toeing Out found the better alignment. This can be discovered as a selected placement where reviews are done on B&W Diamonds where those with the speakers are familiar with other Speaker Designs.       

An improvement put in place in a particular environment for mounting a Cabinet Speaker, is in my experience usually ubiquitous. The benefits discovered will transfer to all Cabinet Speakers used in the same environment, as long as the Speaker Weight is compatible with the chosen mounting method.

There is much to be read/discovered where the benefits of mounting methodologies adopted have brought something that has quire an increased attraction for the end sound int the home audio system.

Any monies spent on alternative mounting will not be wasteful, as it is future proofing. There is the strong likelihood that the Speakers present in a way that is an increased attraction to you. At the upper end of options that can be considered, only one eighth of the $8K may be needed to be spent, if items of interest can be sourced used, maybe one sixteenth of the budget will be used, if such a purchase method is agreeable.

The Link brings to the forefront a selection of options for Speaker Mounting that can be considered,

https://forum.audiogon.com/discussions/townshend-audio-podiums-the-full-review

If you would consider a powered bass section might look at the Paradigm Founder 120H. Very detailed and textured with bass that you can feel. Once you run Arc for room correction you can also make some adjustments. I have a 2db boost I put in at 80hz. 

A used pair of Von Schweikert speakers, generally easy to drive, generally no flaws in the sound and great dispersion.  For half your price, the VR35 export would be great.  For 1/3 your price, a pair of Legacy Signature IIIs (1990s) would be great (rear tweeters for great dispersion and 3 10" punchy deep woofers, 30 tube watts sufficient).  For $8k, maybe a higher end older used Von Schweikert.