The Sound Devices preamps (USBPre2 and MixPre series) are all excellent boxes but there are a few things to consider. If the used V3's that regularly come up for sale are too expensive for your budget then the SD boxes are likely to be as well, even used. As jbell pointed out, you will also require a preamp with digital out if you want to run 8 channels on the DR-680, which would take the older MixPre (the non-D), Audioroot Femto, and Little/Tinybox out of the running. And while I believe that the USBPre2's S/PDIF output plays nice with the DR-680's digi-in, the MixPre-D utilizes AES and I think there's been some grumbling that it doesn't always digitally lock with the DR-680. YMMV.
To echo what a few other folks have been saying, the DR-680's pres are very nice and I'm not sure running another preamp, analog in, is really necessary. Might as well maximize the number of channels you can run on the Tascam if you're going to spend money on an external preamp. Just my .02.