Center for B&W DM603 purchased in 1997

I purchased a pair of DM 603 speakers in 1997. I have used the speakers to listen to CDs, DVDs, and TV. The receiver was a basic Yamaha stereo receiver. A few years ago I purchased a new TV and a Yamaha RX-V871 receiver. I am considering upgrading with a center speaker and then possibly a subwoofer.

The main reason I want the center channel is that I have to turn the volume up on movies to hear the dialogue. Then I have to turn it down when there are special effects.

The brochure for the DM 603 speaker includes the center CC6. I can buy at Best Buy a HTM62 S2 center. I am concerned it may not match the DM 603. Since I have never had a center speaker I’m not sure if I can even tell if the center speaker is matched.

The receiver has a Yamaha Parametric room Acoustic Optimizer (YPAO) function to optimize speaker settings. Will YPAO help if the speakers are not matched?

My concern with buying the HTM62 S2 center is that it is not matched with the DM 603 speakers. My concern with buying a used CC6 center is that it will be over 15 years old and I’m not sure how the previous owner cared for it. If the used CC6 was stored in a garage for a year or so, I may not be happy with the sound.

I would appreciate any comments.
Before you do anything, I would make sure that your current receiver is set to stereo and not surround. If you listening to it in surround mode, you won't be hearing a good portion of what's on the movie because your processor is sending some of the signal to channels that don't exist. Also, look for a feature called volume normalization in the manual for your receiver. Its meant to fix the exact problem you are having.
"The main reason I want the center channel is that I have to turn the volume up on movies to hear the dialogue. Then I have to turn it down when there are special effects."

A couple of cautions here. Based on my experience, a center speaker may help a bit, but I wouldn't expect it to help a whole lot with regard to the issues you mentioned. Thus, I'd recommend you find a dealer who will allow you to try one out in your system and see how much it helps in dialog intelligibility. I have played around quite a bit with two different receivers with and without an actual center speaker.

First, if you have gone into the speaker setup in the receiver Menu and "told" the receiver that you have no center channel speaker, the receiver is already sending the center channel signal to your front L/R speakers. However, if you do get a center speaker, you will be able to raise the level of that speaker, but you can only raise the level of the center channel so much without screwing things up and I never found that to help all that much.

Second, with respect to having to turn the volume up to hear dialog and then down when bombastic effects occur, that is just the way these movies are mixed today. Most receivers include some sort of dynamic EQ or "night mode" feature to mitigate that a bit, basically reducing the dynamic range of the sound track.

Third, yeah finding a good center speaker is a problem when you front mains are fairly old. I guess you could find a stand alone B&W dealer and they may be able to help with matching. As to the YPAO, yes, it will still work with whatever center speaker you get, as it basically is setting volume levels.
I bought an old BW center for about $100 on audiogon

there is the cc6 and another one that is higher up the food chain