Is an unbalanced preamp a bad thing?

Voice from the past...RCA connectors have been working fine since the 1940s. I'm going to guess it's got a few more decades left in it. 

😀 XLRs have been around almost as long- back in the 1940s though they used barrier strips.

Absolutely do not worry about it. Balance or not balanced is a design choice… I have had both and intermixed them. Typically there is virtually no difference in sound quality… Conrad Johnson components are incredibly good sounding… I would not think another moment about it.

I own Audio Research Reference gear and have tried to tell the difference between single ended and balanced… and could not.

Conrad Johnson is one of the greatest audio companies in the world producing some of the most musically satisfying components available. Enjoy!

Is an unbalanced preamp a bad thing?

its terrible, safety hazard! have fire extinguisher and nomex suit ready....

Some people actually prefer RCA over XLR. Keep in mind, just because there is an  XLR connection, it doesn’t mean it is truly balanced. 

It depends.  If the balance of your gear is fully differentially balanced then you will typically find some degradation in sound quality if you use the single ended inputs and outputs.  Examples of this where it really makes a difference are brands like BAT, Ayre Acoustics, Esoteric, etc...  If your system is not differentially balanced, the gear simply offers XLRs, then usually you will find the RCA connectors will sound as good or better unless there is a bad noise problem.