Quick answer to the power supply is yes you need an supply with more voltage.
As wire length or Diameter increase more voltage is needed to push enough amps to heat the wire to the cutting temperature.
I suggest to make or buy an adjustable supply of minimum of 50 or 60 volts. Commercial supplies of this size are very expensive.
I use an home made supply of aproximate 8 amps 50 volts, controled by a light dimmer, to control the voltage to the wire. My bow is 54 in. long with .014" Rene wire and it takes about 28 volts maintain 1.2 to 1.5 amps cutting temp.