how do you select a center channel to match when the company only makes monitor speakers?

I have had dealers recommend using a single speaker from same speaker line. I think that makes the most sense as well. Obviously it won't be a traditional center channel speaker, but the voice matching would be, and that is what you're after. I eventually will be in the market to replace my speakers and have decided that I may be doing that as well.  I have a dual (2 channel/HT ) system, and I want to buy speakers for sound quality in 2 Channel, as it appears you are doing.

I would try to see if you can buy a single Harbeth.  Maybe a smaller model within the same line...... It's unlikely another brand will be voiced similarly. ....

I would try to see if you can buy a single Harbeth.  Maybe a smaller model within the same line...... It's unlikely another brand will be voiced similarly. ....

Agreed