Krikey!
How big is your room that you want 4 18" subs, man?!!!
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Don't get me wrong... I love it, in theory, but I think I experienced some hearing loss just thinking about it!
But I agree with Lovin... depending on your budget, and if you want to buy new-new, you want at least a Marantz SR6013, maybe 7013. If you go separates, AV7705. Denon is the same parent company, and very similar. Their AVRs have slightly less gadgetry, but a little more robust amplification. Just compare models comparable to the Marantz offerings. Last I looked, they do not offer a Surround Pre-Pro like the AV7705.
I've seen a lot of chatter about the Yamaha Aventage line. No experience myself, but its getting some raves. Anthem is on my list of future possibilities, but a bit pricier.
(Currently I'm on an SR6012. I dig it, though I've had a few issues with my unit requiring some warranty service. That said, it seems to be working now.) As soon as I get my new speakers, I'm going to start exploring the world of 5-channel audio. Previously, I had it connected to my old Onkyo HTIB speakers from 2008 and needed to listen in all-channel stereo mode just to get the SPL to enjoy the music. It does work.
That said, keep your options open: get the best, matching front three you can. and if you want to explore 5-channel audio, make certain your L/R Surrounds are pretty decent and fuller range than the normal rule of thumb (~80Hz at the low end).
Cheers, and Happy Hunting!