Effects of concrete as base for subs, speakers and potentially equipment

If your friend is good with shapes, insets, etc. you have a few interesting options:

• Concrete Mixture - Check with your friend whether he thinks that adding polystyrene balls (sometimes added for a lower unit weight) would help with damping and maybe reduce any ringing.  You can also consider adding a color conditioning admixture to achieve a certain look.
• Mass Loading - You have the option of adding threaded embeds (or through-holes) so you could rigidly attach your speakers directly to the concrete bases, which would be a form of mass-loading similar to the effect of using Sound Anchor bases.
• Stability - If the speakers were attached to the bases, not only would the added weight and lower center of gravity improve stability but you could further improve stability by extending the bases out beyond the speaker's footprint.
• System Damping - With the speakers attached to the bases as described above you could then use decoupling type footers beneath the concrete base for optimum performance on a suspended wooden floor.  Individual springs or Townshend pods would be an interesting choice and if your friend could make insets in the bottom you could have the effect with only a small portion of the spring being visible.  Another option that I like is to use platinum silicone elastic footers, which basically operate as (elastomer) springs but are much easier to set up.  I have these under my 150 pound subs and these under my 180 pound monitors and stands.  They come in a range of sizes and support capacities so you can use them for virtually any size speaker or component.
• Base Only - The easiest (and least interesting) option would be to simply set the bases on your floor and set the speakers on the bases using the speaker footers of your choice.  You may want to use one of Herbie's options (like Giant Fat Dots) between the bases and the floor or interestingly you could use a sheet of something like Owens Corning Acoustic Board between the concrete mass and the floor.  It comes in one or two-inch thicknesses.  BTW, I believe the well-regarded AV RoomService EVP damping supports may be made from Owens Corning 703 (for the medium) and 705 (for the hard), either of which could also be considered beneath your concrete bases (you could cut the boards the same size as the bases and paint the edges of the yellow fiberglass material as desired), or you could simply buy some EVPs.
• Equipment rack or slabs.  No reason not to use concrete bases under equipment or use concrete somehow as an equipment rack.  I would probably decouple components from the concrete using one of the methods described above.

Good luck - post some pictures.

I limited my earlier response to your question about using custom concrete slabs and did not address the floor. The wider the slabs, the greater the improvement by spreading the load over a larger area, but @o_holter is correct that gains can be made by reinforcing a suspended wood floor from below.

The easiest way to improve that situation would be to stiffen the floor by attaching a simple, adjustable jack-post column beneath the floor joist nearest to the two speaker locations and possibly also near your equipment rack, as @o_holter did near his turntable. However, not everybody has the ability to feasibly add support columns in their lower level, depending on the usage of the room below where your system is located. Some folks have reported an improvement by adding cross bracing between the floor joists in the vicinity of the speaker positions, with or without the support columns. If you do add cross bracing, be careful not to affect the integrity of the floor joists by cutting or drilling into them (consult a builder if you need advice).

In general, structural members are stiffer near the ends where they are supported so the closer your speakers and electronics are positioned to a bearing wall, the less of an issue you should have.