To play devil's advocate for a moment, I'm not sure you need an external preamp or get the 680 modded. Quite frankly, I think the stock 680 sounds damn good and have used its internal mic pres for dozens of recordings.
To judge for yourself,
this comp is a good place to start. It's
effectively a comparison of an external preamp (the Tinybox) vs. the 680 mic pres. Yes, there is the difference of the CMC6s being in the signal chain (and we were originally trying to test the TB vs. CMCs), but it's also applicable to your situation... quite possibly moreso. The end result was that nearly all feedback we received (and my personal opinion) is that both sources were 'essentially the same' and if the the 680 mic pres were grossly inferior, I don't think the results would have been so close.
In addition to the interesting results of that comp, I am not aware of a single modded vs. stock 680 comp and without anything to listen to, I simply can't give any credence to the benefits of a mod. Maybe it's a night and day difference, but spending money on something I have never heard is not something I like to do... ymmv. Keep in mind that it requires time to have the unit modified, voids the warranty, and forces you to rely on a 3rd party vendor for future support. Tascam isn't going anywhere and can service your deck, under warranty, should you have issues.