How to amplify four 'front- speakers

Newbie here. I have an old Sony DA3ES Receiver driving an equally old pair of B&W DM640s with B&W 303s for side-positioned surrounds. The receiver has connections for only two 'front' or 'main' speakers so the 303s are hooked up as surrounds. Is there a way to get the signal going to the mains to also travel to the 303s?

Not safely. The 640's specs show that they are a 4 ohm load, while the 303's are a 8 ohm load. Doing the math shows that your Sony receiver would 'see' a combined 2.7 ohm load, which it is probably not strong enough to drive.

In fact it would take one amp with a VERY stiff power supply to drive both those speakers at the same time, an amp that is stable down below 2 ohms.

You could use an Adcom GFS3 or a Niles speaker switcher box.

http://www.ebay.com/sch/i.html?_trksid=p3984.m570.l1313.TR0.TRC0&_nkw=adcom+gfs3&_sacat=0&_from=R40

Thanks for your help, Mofimadness. Would the Adcom speaker box be able to differentiate between the 4 ohm and 8 ohm speakers and drive all four properly, or do all speakers connected to it need to be the same impedance? I love my 604s but finding two more is no easy task.

The Adcom GFS3 has a "protection" button on the front. When in use, it keeps the impedance at around 6-8 ohms, so the amplifier is protected.

I have owned a few of these and they work very well. There is also a GFS6 that does up to 6 pairs of speakers. The GFS3 does up to 3 pairs of speakers.

I have a pdf of the owner's manual I can send you, if you would like.

Thanks again...this looks to be the way to go. I was able to download the needed owners manual but I appreciate the offer!

Right now, with the 4 ohm 604s hooked up as the mains/fronts and the 8 ohm 303s as the surrounds, the sound is good. Having said that, would I expect to get better performance using the Adcom box where the main signal will go to all four speakers instead of just a surround signal going to the 303s?