Where to start?

I've never heard a spec (but I'm sure someone will say they have). You need to get your ears on some speakers to make an informed decision. Just my humble opinion. Good luck.

Alot of info if you do a forum archive search "bookshelf speakers"
Good luck!

Is your budget around two grand ? Are you going to place speakers on a book shelf, or do you intend on buying dedicated stands?
Are you using them just for music or for home theater as well? Are you willing to add a subwoofer?
What kind of music do you listen to? How big is your room? Do you want to listen at real concert levels, or at medium and lower listening levels.
Are you willing to buy used?
These are important questions to get a better understanding of what speakers will best suit your your needs.
Todd

You have already started. You said you listened to the B&W CM5. So this is your baseline.

How did you like the sound? The CM5 is a rear ported speaker so that tells you it should be used with stands. It also has a metal (aluminum) dome tweeter. So now you should listen to another 2 way bookshelf that has a different type of tweeter. Listen to a soft (silk) dome, then maybe a ribbon, then maybe a compression horn. They all sound different from one another. Find the one that you like best and zero in on models that have these types of high frequency drivers. Now decide if you want rear ported (stands required) or front ported (a bookshelf or stand can be used). Finally consider the impedence of the speaker. Are they 8 Ohm (easier to drive) or 4 Ohm (requiring more amplifier power). This will help you decide on an amp.

As you audition speakers, based on above criteria, you can begin to develop your own spreadsheet as you narrow down what is the best speaker for you. Good luck on your journey.