If you are looking to finally work your way up to a balanced system, then looking at balanced amplifiers and initially using them single ended may be the way to go.
Look into how the 55se handles a single ended input. Some may say it works great, some may say it’s not so hot compared to other attempts to do the same. Looking into it will let you in on those details.
In many cases, the RCA jack and the balanced jack on an amplifier may simply be the manufacturer making those connections for you, ahead of time, instead of you having to purchase a RCA to balanced cable.
There are a few methods/ways to do so, regarding the given amplifier’s input circuitry. Some methods inherently sound better than others. Some involve extra circuitry right at the input (ie, cheap pro gear) , which is, generally, a bad thing.
BAT seems to be trying got keep the various possible compromises out of their gear and make you use a cable to get it done. For whatever reasons they may have.
The BAT 55se is balanced in fundamental design, so it would take the single ended signal and handle it in a balanced amplifier type of way. That is all it can do.
Their manual says to use their balanced conversion cable, which may simply be a bog standard RCA to balanced cable.
One way to convert single ended to balanced, is to use transformers. The right transformer in the right application, can be magical (sounding).