Hi Rich,
Whether the 6100 is for you depends on what you're looking for from the amp. I've played several (don't own one....still on the lookout for the right one...) but it's a different kind of amp than the Vintage Modern. There are lots of 6100 fan(atics) here, and they'd be in a better position to provide and in-depth review, but in my limited time with them, I see them as providing most of the flexibility (and in some cases more) of the JVM, but with more of a vintage vibe to it (particularly in channel 1.) The VM on the other hand is a single channel amp...and while it has modern features (effects loop, MV, etc.) it's not as easy to control live unless you embrace old-school techniques. The 6100 does have an effects loop and a bunch of other features:
http://www.marshallamps.com/downloads/f ... %20hbk.pdf
Build quality so far as I can tell with the 6100 is the same as with any other Marshall (and same as with most other big brand amps out there....) In my experience looking for one, I've found that because of the features and flexibility (and particularly when you consider when these things came out 20 years ago) most of these have seen some road time. That's not a knock against the amp, more of a testament to how they're used.
In any case, you really should try these. Descriptions from bums like me are fine if you're trying to limit the field, but from some of your posts here, it sounds to me like you'd do yourself a favor by trying to find some of these locally to test drive.
Out of curiosity, is the Shiva lacking in some way for you? It's no slouch at putting out vintage Marshall tones.