Over the last couple of nights, I've of course been watching this thread closely so that I can make the best decision possible up front hopefully without spending extra money on a decision that I will regret a couple months later. Obviously, I appreciate everyone's feedback and if you go back over the threads there is a consistent theme.
****Try everything out yourself and reach your own conclusion based on what your own likes and dislikes are.****
To that end, instead of investing my own money in testing everything, the next best option is to spend some time with existing tapes. So, I've spent spent at least four or five hours the last couple nights researching this question on Live Music Archive. My methodology was to download two songs from at least three shows each from all of the most popular MBHO-preamp combinations (six songs from each MBHO-preamp combo). Then I compared each of three shows to get a general idea what are the specific aural characterisics of each preamp with the 603a's. Then using at least one of the files that represented the most representative, I compared between the different popular preamps.
I'm at work right now, so I don't want to provide details from memory, but there are a couple of general conclusions I can provide right now.
Conclusion 1)
The combination of the 603a's with a specific preamp had fairly consistent aural characterstics from venue to venue and recording to recording, In other words, while it's obvious that the venue, mic location, etc has a significant amount to do with the quality of a specific recording, that doesn't necessarily influence all that much the aural characteristics that result from the combination of these mics with a specific preamp. The general preamp characteristics were fairly consistent from one rig to the next. (very bright...warmer...not as defined...etc.)
Conclusion 2)
These mics sound quite nice though all of the preamps that I listened to. So, although I'm new to the world of MBHO's and haven't done a comparison of these mics to other comparable products, based on this listening experience I do have a healthy respect for these mics through most anything you throw at it. However, there are definite sonic differences that are clearly dependent on which preamp you use. Based on the consistency I discovered in Conclusion 1 above, there are also some reasonably defined conclusions that can be made about what can be expected in terms of sound resulting from a specific combination. Knowing what you like in terms of sound, those general conclusions should be able to help hone your preamp focus.
(FWIW, based on this excercise, I'm pretty sure I now know now what I want to buy.)
When I get home tonight, I'll post the specific thoughts and conclusions that I have on each combination.
Steve