As far as affordability goes, I would do exactly what you are planning (I did the same thing as well).
I originally had an Onkyo/Integra reciever. I then went to a B&K Ref 20 pre/pro and a NAD 5 channel amp. And most recently I obtained a VERY nice Odyssey Stratos 2 channel amp for the mains and now use the NAD amp to power my center and rears. This setup works killer. My stereo music is now the best I've heard and the movies are even better than ever.
No matter what, I would get a sub if I were you. I don't know of many main speakers that can go below 20hz. It also takes a big load off of your amp and speakers if you get a powered sub for the LFE signal.
I originally had an Onkyo/Integra reciever. I then went to a B&K Ref 20 pre/pro and a NAD 5 channel amp. And most recently I obtained a VERY nice Odyssey Stratos 2 channel amp for the mains and now use the NAD amp to power my center and rears. This setup works killer. My stereo music is now the best I've heard and the movies are even better than ever.
No matter what, I would get a sub if I were you. I don't know of many main speakers that can go below 20hz. It also takes a big load off of your amp and speakers if you get a powered sub for the LFE signal.