Is an unbalanced preamp a bad thing?

I agree with @ghdprentice for the hifi journey.

In the same time, people in different period do different things, there is thread talking  " After years separates,I'm going to  integrated "

That said ,some people explore more and some one need easy solution. And It might be the same guy in different period.

I have a Herron Audio VTSP-360.  Keith doesn't put XLR connections on any of his gear.  It sounds fantastic.  Before that I had a Modwright LS 100.  While it had XLR connectors, it was not "balanced".  It did however also sound great. 

How does your system sound?  Another preamp might sound better than the one you have, but whether it is balanced or not is probably not going to be the reason it sounds better.

Op - "I am borrowing a Conrad Johnson preamplifiper and it sounds very nice but to my surprise I couldn’t use XLR cables"

Seems like you knew this last year when you started the below thread 8/2021

@facten

i take this poster’s comments with large, very large grains o’ salt

either a very dull pencil, or a professional chain jerker... (or both)...

I think OP has a big question mark in his brain and that  let him to ask again. Very similar with me.

On this forum, people said it again and again ,it is the better choice to use XLR input, but why there is so many RCA  pre or amplifier on the market, especially for some main Street brand like CJ.

In my personal experience, RCA connection bring very good experience, even better than XLR.

All in all, it goes down to the design of the amplifier, the connection won't decide the sound in large percentage.