advice about mic-ing please?

I've never amplified that kind of ensemble, so I'm just guessing. But I'd guess that a condenser placed above should work fine. I'd place the stand behind the performers with the boom over their shoulders. You need some altitude because the C1000 has a pretty narrow pattern. Although seemingly counter-intuitive, the 58 or 57 might even turn out to be a better choice due to their wider pickup pattern and softer highs.
 
When experimenting with mic placement I'd start by pointing it at the cello, since the violin radiates upward, and the horn is louder. I'd take along a pair of headphones so I could hear more clearly what the microphone is picking up while positioning it.

That is the kind of event where people, for the most part, are quiet.   So, I'm thinking that even with a large number of people there, the music from those 3 instruments should be easily heard in a small to medium sized hall.
 
You didn't mention if there is a raised staging area for the musicians. Playing at floor level might present some issues preventing the sound from carrying properly, but if the musicians are on a stage that places them 2 feet higher, the sound should carry well above the audience.
 
If a mic is used, I would opt for a nice condenser mic placed in the center of the musicians as they set in a semi-circle, much as they used to do in the first recording studios. One mic in the center and everyone gathers around it placed slightly higher than the instruments. 

yapweiliang
Dear all, thank you very much for your thoughts.  There is no stage, so we're at floor level.  I'll definitely bring the headphones, and experiment a few mic positions - I think we'd aim for subtle amplification if at all.

It would be nice to have things already set up so that if you did need reinforcement once everyone has arrived, you'd just have to turn it on and adjust.
 

I would try a condenser set to Omni, about 2 or 3 metres above the musicians, if the musicians can balance themselves. If one instrument is louder than the others (I'm thinking the horn) then a cardioid might be a better option as you could point it away from the louder instrument.

I don't know if your're familiar with the C1000 or not, so you may already know this...

Overall, they're a good "overheads" kind of mic, though a bit "different" in a couple of ways. I've been using them for various jobs since I first got one in the early 1990s.

The C1000 has two pickup patterns, cardoid and a tight hyper-cardoid. The hypercardoid setting is obtained by unscrewing the grille and putting a small clip-on circular adaptor disk on the rubber-mounted mic capsule. For a single overhead, cardoid is the way to go I would think, so checking the hyper-cardoid adaptor's not fitted might be an idea. Just be careful removing it as it's possible to tear the capsule support with rough handling.

Another thing to watch with the C1000 is it can be a bit brittle and bright so might need a bit of treble taken off. It's also a pretty sensitive mic, so angling it away from the audience and any speakers is pretty essential, especially as they can go into feedback surprisingly easily. They also have quite a marked bass-proximity effect.

They also run off either an internal 9V battery or phantom. 48V phantom works best with them in my experience.

Overall, I think you'll probably get the best results from the C1000 as the Shures are less sensitive and getting adequate gain might be an issue. They also sound a bit muffled compared to the AKG.

As for balancing the instruments, if all else fails stuff a few socks into the French horn :-)

Dear all, thanks for more tips.  My concern here is primarily the overall volume in a full hall - not want it too soft for the people at the opposite end of the hall, nor too loud, certainly don't want feedback from the loudspeakers; the musicians can balance their own relative volumes.  The do-not-mic option is obviously the simplest option, but I honestly do not know how sound-absorbing 100+ people in a fairly full hall with soft chairs and carpet floor will be.
 
Maybe I'll set it up to record, and make it such that the PA operator can route some of it to the main speakers if it really sounds too soft and leave well alone if loud enough - then I can upload it for you all to play spot-the-melodyne...
 
Wei Liang

Dear all - in the end we did not amplify this, and it worked out just fine.  Am grateful for the advice given, as it helped me to be more confident about not amplifying in this situation.
 
I did try to record the performance in any case, with an overhead C1000S, pointed toward the cello (or so I thought); and that picked out the violin and the horn well, but the cello - not so well, particularly the lower notes.  Probably with more experience about mic placement, etc I could have done better.
 
Wei Liang