Where would the Oade concert mod PMD661 AD/ Pre, fit in this list?
V3
V2
UA5
AD-20
It would not really fit at all into your list, since none of these are all-in-one recorders.
That being said, there are still some comparisons you can make to help you decide on getting a modded 661. According to Doug, and as I mentioned earlier in this thread, the concert mod shares the sound quality of his M248 preamp, and the warm mod shares the sound quality of his M148 preamp. If you collect recordings, you must certainly have tapes where each of these preamps are used and that can be your preamp reference.
Most of the flash recorder brands share the same 24 bit AD chipset within their model line, so that portion in the 661 is likely exactly the same as the one in the PMD671, which is a really good A/D, and there are a ton of recordings in circulation to listen to how that A/D sounds.
Technically, the V3 is the only 24 bit preamp A/D combo on your list, and still a great way to go, paired with any 24 bit recorder with a digital input, that can be used as a bit bucket.
Good Luck!!