Subwoofers - Front Firing or Down Firing - Which Sounds Best?

noromance: I know you have a lot of concrete in your rack. But if it's sitting on bouncy floors, what then?

Well what then is mass equals inertia. That much mass simply cannot be moved very fast without a great deal of energy. Way more energy than a bouncy floor. That much mass can only be moved slowly. The effect of massive concrete and damping sand combines to drive all energy into a very low fundamental resonance frequency range. You can see this in action. Stamping or jumping on the floor does move the rack. But it moves very slowly, and very subsonic, around a few Hz.

The response of this massive rack on a suspended wood floor is actually better than a lightweight rack on a solid concrete floor. Because the lightweight rack is subject to vibrate from acoustic energy. Also I lived for years on concrete enough to know even concrete slab can still move and if it does the lightweight rack will transmit that energy right up to the table. A massive rack will absorb and dissipate that energy into its mass.

So that's what then.
btw, I never have recommended "using any old subs". What I have said is they don't need to match, and more is better than few. Which is true.

Love my down firing REL T/5i. Everything it’s cracked up to be. Hi-level Speakon input, 120 W A/B power with precise rotary click control of volume and roll-off, corner mounted, with a unobstructed plane of 100 year old maple extending 20 ft in front of it. Super fast long-throw lightweight 8" speaker.

The REL compliments my towers, creates a base source that is centered in the sound stage between the towers despite a distant position on the sub. I am amazed at how close this experience is to John Hunter’s claimed performance. Great company, great products, IMO.

The fact that the listener can’t locate the sub once everything is adjusted in the room is magic to me.

     Since I use physically time-aligned B&W 803's, I have my subs positioned to be in phase with the main speakers.  To do this, I measured the center of the B&W magnets, which are all the same, and the subs, then measured where to place the subs.  Of course, I then had to determine whether to place the subs inside or outside the speakers, and finally decided on under them. It worked the best, largely because my speaker room floor is several inches lower than the living room into which it is open through a 12' x 14' hole in the wall (Actually, a lack of a wall).     Otherwise, I would have to use a high chair, but I might just have done that, but probably would have kept them between the speakers.

I find down firing subs are good for 
Home Theater and Front firing subs 
are good for two channel music.

It depends on the driver. Drivers with stiff suspensions can fire downwards fine. Those with looser suspensions are better off front firing as they may sag mounted downwards and the driver becomes non linear. Given a driver that can go either way there is little if any difference. More important is that the driver have one edge right up against a wall or corner. Then multiple drivers and finally room control. The radiation pattern of the sub should match that of the satellites. Point source speakers should have a point source sub system. Line source speakers require a line source subwoofer system. A Line source sub will dominate a point source satellite and a point source sub will get lost under a line source speaker.