i have two (bass) amps [that i use for voice, synths, guitar, and bass] fronting me from the other side of the room, opposite corners (as in left-and-right) – with all troubles happening in between.
fare's too long for unbalanced lines, to start with – then you have signal source, amp left, and amp right plugged into different AC mains outlets... ground loops, hi-freq interferences, unsymmetrical grounding, you name it.
benefit one: instead of taking source signal at the unbalanced, 1/4" l+r outputs of my Helix Rack, i'm now running two long (balanced) cables plugged into its 3-pole XLR outputs – the end of each going straight into the Lehle P-ISO XLR·TRS; a short, made to measure jack-to-jack cable feeds into the amp input.
where's the benefit: there's a superb 1:1 audio transformer inside the Lehle P-ISO, which not only sets a most convenient impedance value across the line, but serves as a galvanic isolator, too – meaning that the signal ground is physically separated from the mains grounding of the three devices (farewell ground loops' hum and interferences).
where's a(nother) benefit: as any normal 2-pole 1/4" jack into the P-ISO output shunts the inverting leg of a balanced line to ground, signal voltage is exactly halved (which amounts to -6dB) – so any line level signal (like the Helix Rack balanced outputs, and my Mackie Onyx mixer's, too) can more comfortably seat within a safe level window that won't saturate the amp's input stage, resulting in a smoother, wider range of volume control.
problem's solved, system can now alternatively accomodate a small mixer, too, and the amps' volume fits exactly where it belongs (on the -10dB "Active" input settings)... bingo – Lehle!
