2-channel pre-amp and dedicated processor?

What type of HT processor are you looking for? i.e., DD,DTS,Pro-logic - some, all, etc.. The suggestions above may work (most will) but it will be dependent on the processor you go with, which is dependent upon what you want in a processor. I had similar issues of integration in the past and I may be able to help. Can you shed some light on what you want the processor to do?

Hmmmm... You have a similar set-up to one I had several years ago with the exception that I had pre-outs and main-ins on my preamp. With your current preamp you have no pass through which pretty much means you have to put the procesor in the loop prior to routing to the preamp. This shouldn't be an issue as almost all stand-alone processors I am aware of have a bypass mode which shouldn't degrade your signal going to the preamp. In this configuration you'd have all channels route to the processor from your DVD player and the the main L&R going from the processor to your preamp. The only problem with this, which others have mentioned, is having to keep your volume control on your preamp turned up when doing HT to match the other channels (the master volume at this point being controlled by the processor).
A second option would be to get an input selector box and put it between the DVD player and the processor (I believe Niles audio makes one of these). It would require 2 sets of IC's for the main channels, one routed directly to the processor and the other routed directly to your preamp. In this configuration you would have the normal pair of IC's from the main L&R channels of the DVD player to the Niles box which would have two sets of L&R outs. One set goes directly to the processor and the other to the preamp. When you want HT, select "Output 1" on the Niles box and when you want music (controlled by you preamp), use "Output 2". You will have to look harder for one of these as a device like this is more commonly found where you have multiple inputs with one output than the other way around as I am proposing. That said, it should work, be fairly inexpensive and provide a clean signal path.
A third option, which I'm not sure you want to consider, is to get a DVD player with internal processing. This would be easier to wire as all of your IC's would route directly to amplifiers with the exception of the mail L&R's which would go to the preamp. The problem here would be to find a DVD player with a variable level output for all the channels - in effect using the DVD player as the processor AND preamp. This would be a pain however and you'd probably have to tweak those variable outputs all the time!
All in all, I would recommend the 2nd option I presented. You may want to check out this link for Niles Audio. If they don't make it, there should be some manufacturer that does. This may do the trick for you. http://www.nilesaudio.com/products/source_switching.html
Good luck no matter what you decide! - Tony