Right. The TPA3118 boards do not need +/- power, just single ended. Ideally I'd like to end up close to 24V DC or even slightly higher.
The transformers have dual primary winding and dual secondary winding. I need to measure them again, I can't remember if they were 30V each on the secondaries or 35V. Something in that vicinity.
Using the + and ground, then - and ground will not work (the input ground to each of the amps will screw things up). I was thinking of 4 rectifiers.
I believe you are slightly misunderstanding how a transformer works.... there is not necessarily a "ground" unless you connect one. By "not necessarily" I mean that the magnetic coupling between the primary and secondary in the transformer provides isolation and does not pass DC. For example, it would not matter whether you connect gound-centere-100VAC (think of a sine wave where the positive and negative peaks are at the same magnitude but opposite sign) or a sine wave voltage oscillating between 1000V and 1280V (1280 = 1000V + 2*1.4*100) the result is the same on the secondaries.
On the secondary side, there is also no "ground" per se - it's floating. Only AC voltage is induced into the secondary. For example, given ONE secondary (two wires), after rectification and smoothing (via one or more capacitors) you can connect either wire to ground to form a positive or negative voltage with the other terminal.
What can happen, and what might lead to confusion, is with a center tapped transformer the two "ends" of the secondary winding are moving current in opposite "directions" but with the same magnitude (they are just ends of the same wire after all) and the middle point is therefore at a midway potential. Because the magnetic coupling does not pass DC (only AC) this results in a center tap voltage that is 0V (ignoring unequal loading, etc), but you could just as well connect it to some voltage reference and it will be happy to sit at that voltage as well.
Your transformer has TWO secondaries. If you connect a rectifier and smoothing cap to each you have TWO independent supplies, however, they don't know about each other or about "ground". You can connect these together in different ways, for instance to make a dual polarity supply, or you can use them separately without any connection between the two. They are electrically isolated unless you connect them.
One way to make use of two identical (identical voltage) secondaries that might work well for you is to connect the secondaries in parallel. This can be done in two ways - one will give the same voltage as one secondary alone, but can provide twice the current. The other is a useless "backwards" connection in which the voltage induced in the first primary is cancelling the voltage induced in the second primary. Sometimes the secondaries are marked so that it is clear which wires should be paired up. If not, you can measure the AC voltage on the open secondaries before connecting them to determine the correct way to do it.
In your case, I would just connect the primaries in series and run your 115VAC main through them. This will give you half the rated voltage on the secondaries (15V or 17V AC). I would connect the two secondaries in parallel. This gives you one single ended supply. Then connect a rectifier and smoothing cap(s). This gives you a one single-ended smoothed DC supply. Run four separate pairs of wires to the power supply terminals of each of your four amp boards.
Don't bother connecting any of the PS to earth-ground anywhere - it's just not necessary and can cause more problems then it solves. Just measure the voltage across the two wires and connect the one that measures the "positive" voltage to the amp + supply and the other to the amp "-" or "ground". Remember, the multi-meter is NOT measuring the voltage with respect to earth-ground when you do this, it's making a relative measurement. DO connect the enclosure metal case to safety earth with a toothed washer, machine screw, lock washer, and nut and use a 3-prong plug.
