When you run SBD + AUD, are you copying the files to a computer, and then taking out the approximately .040 second delay between the SBD and AUD tracks? If not, that's your problem. If so, how do you go about deciding how much offset is required?
Another thing, and I know this sounds silly but I've seen it happen... you realize that A51's are a side address mic, and the ADK logo is pointing towards the stacks, and the ends of the mics are pointing towards the ceiling, right? I once saw a newbie who didn't know that and used LD's like it was a pencil mic.
I've found my ADK LD's (A51's, A6's, TL's) are at their best when they are close to the music, less so from the middle of the large room. The preamps in your MR8 are probably decent, but not exactly awesome. Trying to guess which is "the weak link" and which upgrade would give you the bigger bang for the buck is a tough call.
Before you spend any money, is there another taper nearby with a bunch of good gear you can use for a test? If you lived near me, I would try putting my V3 between the mics and the MR8. If you put a decent pre in the line and it doesn't make any significant difference, that answers the question about different preamps.
Mics are like knives in the sense that they are "tools" and different tools are better in different situations. I wouldn't peel potatoes with a machete, and I wouldn't butcher a cow with a pen knife. My gut feeling is that A51's are good mics, they just might not be the right tool for that job. My suggestion is to try some AT853's, the standard RX ones with phantom adapters. Most people will say that AT853's and CA14's are pretty similar, but the AT853's will plug into your MR8 directly, no battery box or small preamp required. You can get them used in the yard sale pretty cheap, try them, and if you don't like them, sell them again for little or no loss. If you can't find a pair, let me know... I have 2 pair, and while I'm not really trying to sell them, I could sell a pair.
The ability to run 2 AUD + 2 SBD is a huge advantage. That's a much bigger advantage then switching to 24 bit on a 2 track. A "good" 4 track recorder is going to set you back $500-$1000, which probably isn't in your budget. There have been thousands of awesome tapes made with DAT or JB3 at 16 bit.