For the record, how did the show go? What gear did you end up using? Recordings come out ok?
So, in order to provide some feed-back about the show:
Both the artists and the museum staff were super helpful. I was allowed to mount my gear early (about 45 minutes before the general public was allowed to enter the room).
The show itself took place in a former wine cellar in the basement of the museum. Great location, very appropriate for this sort of event. The event manager told me before the performance that they'd only allow 35-40 people into the room.
The only downside with the location was that the floors in the cellar had been paved with pea gravel. So, every time the artists (or someone in the audience) were moving, there's a crunching noise on the recording. Anyway, I consider this inevitable under these circumstances.
As suggested by the other posters, I used a 4-mic array on a small stand next to the artists to record the event (subcards in X-Y-120, omnis in A-B - spaced approx. 70cm apart). I also shot a HD video version of the performance. My camcorder was mounted on a small table top tripod (Feisol TT-15 mkII) and a Sirui VH-10 video head - which enabled me to use an extreme "worm's-eye" view.
Due to the extreme dynamic range of the music, I decided to use some attenuators between the mics and the XLR-ins of my mixpre-6. The mixpre-6 was switched to custom mode with the front knobs acting as gain controls (rather than faders).
* AUD1: 2×MBHO-KA300NB (X-Y placement) -> 2×MBHO-604 -> 2×Cordial CPM 3,0 FM-Flex cables -> 2×Shure A15AS attenuators (@-25dB) -> SD mixpre-6 (CH 1+2)
* AUD2: 2×MKH8020s (wide A-B placement) -> 2×Cordial CPM 5,0 FM-Flex cables -> 2×AT8202 attenuators (@-30dB) -> SD mixpre-6 (CH 3+4)
The resulting footage has been submitted to the artists, their label and the museum. I might post some samples here later on if/when public circulation has been approved.