A couple of comments
I'd vote for the DMP-3 simply because of what you're wanting it for. The
acoustic guitar and room mics are likely to be 2 channel applications. Now if you're willing to buy 2 VTB-1's (ceratinly do-able) then that may be the way to go.
BUT, and I may be way off base here and I'm sure someone will tell me I am, indulge my rambling for a moment.
Not only have I been in your shoes, for all practical purposes I still am, in that I have to get bang for my buck. What you want are results for your money, and you really don't care how they are achieved, right? What I'm getting at is that you're not really jonesing for a pre, you just want something within your budget that will make you sound better and you're assuming that will be a pre. Yes and no. While either one of those pre's would probably sound better than what you've got, I don't know that the mics you've got will let you hear that all that well.
Not what you want to hear, but I'm thinking a mic upgrade might do more to get you where you want to go right now than a new pre.
Do you have any friends with better pres or mics that you could try out? You know, swap things for a couple of days just for fun? Just make sure you use their pre at your place with your mics, or their mics at your place with your mixer. First rule of experimentation: only change one variable at a time, or it's not a valid test.
Just what you wanted, more to think about.
Have fun, and don't stress about it. If nothing else, buy a VTB-1 or aDMP-3 and if you don't like it, sell it here. There's always somebody in the market for them. You'll take a little hit, but hey, experience isn't free.