I was running a B&K 507 with an external amp (Halcro MC50) amp. That made a very significant difference as the Halcro was much more detailed, dynamic and refined. That's no knock on B&K. They make truly incredible products for the money and I am a big fan. Indeed, the Halcro amp was almost 3x the cost of the B&K reciever.
What you proposes does not strike me as very cost efficient. Using a B&K external amp will get you from a slight increase in power. You will not, however, see an enormous improvement. I believe that B&K that would tell you the same, but I suggest calling them anyway.
A really good set of interconnects and, I mean really good, like the Synergistic Tesla series, would change the equation, but that would run you some serious cash. One way to go would be to upgrade your power cord. A good cord can make a world of difference. You can even make one yourself for not that much money. Check out Chris' VH Audio site for details. These days of course, cash is King and you can get pretty much what you want on the Gon at a big discount.
I would probably start with your power cord. Another alternative is two buy two amps, one really good two or three channel unit less expensive one for your front speakers and one for your rears or rears and center. That would be the most musical. A used Halcro 2 channel MC20 is reasonably priced and would rock your world.
i would move to separates anyway. if you intend on progressing with the hobby, you will get there anyway and when you upgrade, you can do so only for one component at a time.
Hope that helps.