Any Home Theater Experts here?

pressuming you're staying in 5.1 channels, and not 6.1/7.1 (lest you add amp and speakers for rear/sides), I'd look at quality over quantity indeed. I'd even sacrifice the newer DPLII/IIx in favor of doing 2.1 channel material for stereo movies and such if needbe.
What you can expect from better pre's is better dynamics, lower noise floor, better channel separation, low level detail, refinement.
I'd start at a minimum of Acurus Act 3 with ABM for a budget. I've a factory upgraded 7.1 Acurus unit with 24/192 DAC's myself that's superb, if rare. Another choice is a used Thule or Cal Audio 2500. Krell HTS is superb and refined for $1200-1500 range used for 5.1, if a tad less dynamic than the suprising little Act 3. Otherwise, considering the Anthem AVM20 or better is a good chioce.
If you have a difficult room setup, where you have problems with getting flat response, I'd consider some of the models with a built in parametric EQ, if you care. It does make a huge difference in a system with wall mounted speakers, and diffucult speaker/seating choices.
Do you need 6.1, DPLIIx, etc? There's lots of other choices out there. Many that won't do much better than what you have now however. The Outlaw 990 probabaly won't sound much stronger than the Sony's preamp section however.
Good luck

Most receivers lack the dynamics, channel separation,signal to noise, and overall low level detail retreival that a good separates piece offeres. That said, for $450 range used, something like the modest Acurus Act 3 offers significantly more potent movie sound than that of the preamp section from a Sony receiver. I've not only owned the 777ES and 9es pre's from Sony, but sold the 777 receiver as well. I know of what I speak. My two cents...

"I would not do the ACT 3 as technology has moved on... I would do a Denon 3806 with room correction and keep you current Preamp." (cytocycle)

Actually, technology hasn't moved that much! Infact, for all the applications you'll use, nothing we've got right now is really supperior for your movie applications than that old outdated ACT 3! Infact, I've got a piece that uses 24/96 and 24/192 Dac's in it, and it's not really better for DD/DTS movies than my old ACT 3, just a tad quieter in the signal to noise department. Otherwise, my old Act 3 was just as potent!
Look at it this way, everyone's either playing CD's, which are 16/44, DD/DTS movies, which ARE NO HIGHER IN REZ, and analog or Hi rez audio sources (which is covered by your 2 channel setup!). Nothing right now in a processor is going to beat up on the old Act 3, and similar!...just more bells and whistles mostly.
HOWEVER, I will definitely conceed that some of the newer processing for Parametric EQ, like in the Denon, B&K, Sherwood, etc, are VERY VERY practical and effective in most setup's, yes!!! If you can't get your speakers/seats to where you're getting reasonably flat response from your system, then yes, you will definitely benefit from this sort of acoustical help! Still, if you can set up speaker properly, then you can get around this. However, the piece in question still has to sound good on it's own. Receivers are mostly a sonic compromise, with some exceptions (i.e, the Arcam).
If it were my money, and I had a good setup, and didn't want to spend a bunch, I'd not hesitate to stick in another ACT 3 in my system. I've owned 3 or 4 over the years, and did what you're doing, which was loop em into an auxilary, tape loop, or "bypass" input on a 2 channel higher end preamp for music dubties. Works great!
Good luck