Generally speaking, for Doug's preamp mod he rebuilds the XLR input path up to the A/D stage. The input capacitors and op amps are replaced with higher quality components with better performance than the stock parts. The concert and warm mods are optimized for loud sources, and the super and FET mods for quiet sources. There's more stuff he offers depending on the deck, but I think I've summarized his main offerings fairly well. Just do it -
Send that Tascam unit to Thomasville, you won't be sorry.