I have two 1977 Marantz speakers hooked up to the "B" speaker output on my Onkyo 575X. I have the speakers upstairs while the receiver is downstairs.

I also have two old speakers that came off a boombox lying around. If I removed some of the wire covering and spliced in these other two speakers, would I get more volume?

No, you would just be spreading the watts even thinner if you hook up the four speakers.

okay, that's what I thought. Thanks greddy.

No prob.

I think the answer to your question is a function of how much resistance your speakers 'present' to the amp and how much current your amp can provide in return.

Your Onkyo is a relatively 'high current' amp, meaning it is better suited for this than others.
My guess, though, is that you already have two pair of 8 ohm speakers being driven--meaning they appear to the amp as a 4 ohm load. Less resistance means more juice must flow (ie, high current).

Use an ohmmeter (any multimeter has this function) and really see what the resistance is of your speakers. If they're all over eight ohms, your amp may just drive three pairs of them. Look in your manual to see what the minimum required resistance is. There may also be a selector switch on the back of your receiver for this (my Yamaha has one).

umm ... I thought the rated "resistance" is actually the impedence of the speakers -- which is what is faced by an alternating current signal (like an audio signal). An Ohmmeter will not be able to measure the impedence of the speakers ... it will measure only the passive resistance as seen by a DC signal.