Devices like the Aphex are used to match "semi pro" type connections with "pro" connections.
Two issues - output level and balanced/unbalanced.
"Pro" - balanced, slightly higher level, and possibly lower output impedance. Usually on XLR or TRS jack.
"Semi-pro (hifi)" - unbalanced, lower level, usually on phonos or mono jacks. And maybe, only maybe, higher output impedance.
(But generally, with line level connections, you don't need to worry about impedances, because line inputs always have high-ish input impedances and line outputs have relatively low output impedances. Modern active output stages almost always have an output impedance of 1k or less, thanks to the wonder of the op-amp.)
It's perfectly possible to connect pro to semi-pro without one of these converters, people do it all the time when necessary. But you need to know how to wire things (not that complicated), you lose the benefits of the balanced connection, and you may have a slight level mismatch. And very occasionally, you can get subtle earth loop problems.
If your piece of pro gear has an output level control, just turn it down a bit.
In general, any inputs or outputs on phono connectors will be "semi-pro", so you can connect them together and you're golden.
Hope that makes sense.