The isolation transformer would be good for working on SMPS circuits and is pretty much essential.
The linear supplies are useful and can be made adjustable by using a variable regulator, or use fixed regualtors to give 3.3v, 5v (digital), 6.3v (tube heater), 9v (stomp boxes), 12v, 15v, 24v (relay) outputs.
Ideally for a bench supply you want +/- 15v to power op-amp circuits. You may be able to rig this up. 12vac is also useful for Line 6 products.
As for amplification, 24v would power a chip amp.