I've been in almost exactly the same situation as you - with a few significant differences - and wanting to achieve essentially the same thing (i.e. all of my media located in
one room as opposed to two or three). If you've heard Maggies before then I assume you already know their sound - I find it intoxicating to say the least.
The biggest problem with the maggies is going to be amplification. Your Denon is way better than my Pioneer (even though it is the top of the line for the brand in the non-Elite branded line at least). However I would seriously doubt that you will be able to 'milk' them adequately in a stereo pair (let alone 5 of them). You'd probably be better off getting just one pair and a
center, if you can find the place to put it and skipping rears altogether - although using 2 more MMGs would definitely give you a full surround experience.
The issue with them is that not only are they 4ohm speakers (and 3ohm for the center) - but they are not very sensitive so unless you give them lots of power they won't be nearly as 'impressive' as cones will likely sound in the same space (however, IMO they will sound much, much more 'accurate'). The one nice thing - or can be - is that the load they provide is a purely resistive one - so you don't have to worry about them having a dip into the 2ohm range that some 4ohm-rated cone speakers have.
If you're open to upgrading amplification down the road and listening at conservative levels now - I'd say they can't be beat (for the price) in the SQ area.
One other speaker you have undoubtedly not considered (because of the price) but I think you should take a listen to if you have a BestBuy nearby is the
Pioneer SP series. I
think that is the route I'm going for the present time - although I'm still in the "waffling" stage.

I really liked them and for the price they are an absolute STEAL! I'm still fully planning on getting a pair of the Magnepan 1.7s to put in a 'music room' as some point in time - so I've opted to reduce my current budget and make some compromises in the name of saving for the construction of my casita/mancave - which is where the 1.7s will find a home.
You can setup a full 5 channel system with the Pioneers for less than $500 - yep that's right (since you have a sub already). In fact, if you have the room you can do what I'm planning - and get 4 of the towers and the center - and still be under the $500! If you got 4 of the bookshelves (although they're too big to call that - more like "standmounts") then you could do it for ~$350. I will put more into a pair of good subs (~$600-$700ea.) but even then the whole system would be just around $2K.
I personally do not care for their appearance at all - but for the price I don't have nearly as much problem as I would if they were $500/ea.

I know it seems unbelieveable but go take a listen and see what you think. They will probably be setup like crap (it's BB after all) but since you can always return them, you could just pick up a pair - or even the whole set - and audition them in your home for 29 days - and then take them back if you're dissatisfied with them.
I think you might be as surprised as I was! At their prices, I decided to just go with 4 towers for fronts and rears because it would cost me more to buy stands for the BS than it would to just drop towers in the same spots.

If nothing else (assuming that I don't want to have a 5.1 system at all once I have a 'Maggie Room') they're at a price I can easily just make a couple of friends happy that are currently using PC speakers and an iPod as their 'hifi' and not feel bad at all. But I really do recommend hearing them - I thought they sounded better than the B&W's I listened there - at least in most areas. They really are a pretty accurate and reasonably flat (FR-wise) sounding speaker and they are definitely better sounding than anything I've auditioned lately for less than $500/ea. Not saying they're the solution to your problems, or even that YOU will think they sound as good as I did - but it's worth a free listen in any case.
And zieglj01 has another good suggestion - one that I've also considered recently. I've heard nothing but good things about the NHTs - unfortunately I also have heard nothing ON THEM (musically speaking) so I can't comment on them as a comparison.