Tower Speaker/Amp Advice

I am looking to add some speakers to my tower. Without spending a lot of money, I would like to get the best set-up I can.

I found some Polk Audio speakers with double cans on E-bay. I found both the DB651 and MM651 series speakers.

Any recommendations or warnings on these speakers? I have read some posts were people say they aren't loud enough when boarding and others say they are great??

I will also need an amp. Any recommendations here? I am not sure how the double can speakers connect - if there is just one connection for both speakers or two?? So I don't know how to spec out the amp.

I will eventually add a sub and would like to run the boat speakers through an amp as well. Am I better off to get more than 1 amp?

Thanks