Sean is not so far fetched in his post. Really, at that budget, you should do separates, hands down. Even the mighty Denon 5803 and smiliar aren't as good as good separates. And, yes, the amp sections are the largest part. But dynamics, signal to noise, and overall purity suffer in the preamp section there as well.
Also, the gear your mentioning is not that special really! I know, I sell it. IT's "OK" stuff, but with athat budget, you could do much much better. The Yam 2400 is farily plain sounding, especially when you use the parametric EQ inside. The Marantz amp also, just ok.
I'm exteremely familiar with those 2000tl's and center/sub. I've sold them for several years now.
If you're looking used, the Parasound amps gererally do very well i balancing the sound of the Def's! A used HCA1206 or 1205 would suffice just fine, and compliment the sound. Def's are usually a bit cool and slightly hollow sounding (kinda like Yamaha receivers in general), if a bit tipped up(although your 2000t's are better balanced than others in that line) and grainy in the trebble. The Parasound is a bit warm boddied,and forgiving on top, which mates well with the Def's indeed. Also, more expensive Classe and Aragon amps. But I think you'd like the Parasounds just fine...I've owned/sold all over the years.
For pre/pro's, if your looking for 6.1, the Outlaw is going to be hands down the best deal you'll find! There are more dynamic out there. But for what it offers, for $800 new, it's the deal!!! That said, you couldn't go wrong doing all Outlaw separates, as the amp (100wx7 or 200wx7 will put you under your $2400 pricerange too!.and that's all new!!!!
As for other pre/pro's, depends on what connection options you need. The outlaw at $799 has superb DAC's for digital connections, pure analog bypass for better 2 channel, and lots of nice bass management flexibility options(your center channel should still be set "smaller"!!!!!...even though it's powered!..you'll distort it at high bass levels otherwise).
There are lots of great pre's out there, but sometimes pricy used. About as cheep as you'd find otherwise in 6.1 for quality is the used Classse SSP30, Anthem AVM20, Maybe Sunfire Theater grand II/III, and similar with good sonics for decent used pricing. I think. Hummmmm....
Anyway, my vote for a "no lose" is the Outlaw separates.
I say that, and I've sold Everything imaginable from 6 audio stores over the years! The Outlaws is great value and a good match with your 2000tl's