Brad,
As Tls49 mentioned, your Onkyo is a two channel receiver. I believe that you have mistaken the fact that is has two sets of speaker terminals for each side to mean that it is a four channel amp.
The specs for your receiver, as published on Onkyo's website, indicate that your receiver is rated at 100wpc, but they also indicate that your receiver is neither 6 or 4 ohm capable.
From what I gather based on the specs, your current receiver doesn't have a very robust amp section.
What I'm saying is that if your current amp was driving your present speakers, all at once, and it wasn't over heating, then your speakers must not present too much of a load. Either that, or you didn't play them very loud.
Just about any quality two channel integrated should be able to, at the very least, match the performance of your current receiver.
I've been a long time fan of NAD equipment because I think it offers a very good bang for the buck in terms of sound quality and power.
It should drive two pairs of speakers, with a nominal impedance of 8 ohms without problems. It's also going to sound better. What ever you check out, just be sure that it is rated to supply power at the impedance load that your speakers are going to present.
If your speakers have a nominal 8 ohm rating, then running a pair in parallel is going to present a nominal 4 ohm load to the amplifier. I do believe that NAD equipment is rated to at least 4 ohms.
If you just have your heart set on spending the whole $2500, then shop around. There's lots of good stuff out there for the money that you can spend. I'd suggest checking out the classifieds here to see if anything sparks your interest.
For $2500, you could get yourself into something that retails for $4K or more.
