Is it okay to do so? Appreciate your comments.

Planning to buy a used Classe thirty preamp and a seventy amp. Connect them by using XLR cables. Now,can I connect the preamp rca outputs to a powered sub.
From the Classe Thirty Owner's Manual:

"The Classé 30 preamplifier has both regular (single-ended) and balanced outputs. Both are
working at all times, and may be used separately, or simultaneously."

So, yes.
MofiM...Thanks a million for your response. Given your response, I'm perplexed as to volume control. Will it be thru the preamp 30, or the sub's.
Once you get your sub setup and dialed in, you will control the main volume from the preamp.