Thanks fo the replies so far. I'm in Dublin - with one performance contemplated in an exquisite newly designed theatre here in Dublin, and the second, more ambitious out door performance envisaged in Cork in a castle just outside Cork city centre called Blackrock Castle Science Centre:
http://www.bco.ie
The director is a very close friend and we have plans for a space themed event with ambitious sound and music - where we're hoping that BCO's endorsement will enable Arts or Science council funding when we put in applications. We're hoping for really exciting sound and music, large screen projection, and perhaps even augmented-reality visual aspects to the event. A completely immersive experience for the audience and hence the need for very high quality audio playback.
I'm fairly experienced as a composer - I've done music and sound design for about 30 plays, 4 TV documentaries ( RTE ) and indeed 4 or 5 electronic music live events over the years. I'm a physics lecturer as my main job - so I'd class myself as part time professional in music.
My project studio is quite sophisticated - it's two rooms in my home with lots of synths and JBL 4412s and Genelec 1030As for monitoring in one, and K&H O300's run through a Benchmark DAC in the other - and Sennheiser HD 650 - 800 headphones.
I've just taken my time over the years acquiring very good play back - but these are two rooms in my home with no particular acoustic treatment and I make absolutely no claim to particular expertise in mixing / mastering / audio-file work - just a musician who's been given good advice and guidance over the years (including by the likes of Hugh Robjohns here on this Forum).
For this event I will, as mentioned, have spatialised aspects to the work using IRCAM Spat - so a 4 speaker setup is necessary for that - so over all I think I can design the music and sound program with good fidelity.
The goal is then to be able to relay / perform that in a live setting with good control and quality.
While of course I will, when the time comes, seek professional help - I have to be frank and say that I have rarely come across an audio company I've been impressed with.
As mentioned above I've been to plenty of concerts in the UK and Ireland in supposedly excellent venues with excellent artists, where too often the sound is not good. For most of the Tangerine Dream concerts I've been to - including in the Royal Festival Hall - the sound was really poor. Even Jarre in the Wembley Arena was pretty average.
And for the live events I put on I've without exception been very disappointed because the audio companies I've come across (some with supposed top notch reputation) seem to have fixed expectations on what can be delivered live - and that usually far below par in my opinion. I'm not in any way being elitist in this - it's just that to me even supposed top quality live audio is usually far below the quality we hear in our studios or over our hi-fi units - and not because of scale or venue specific issues - because of poor PAs and amplification, poor mixing, a 'norm' of it being a money earning exercise for the company in question so they don't really care, and because you never have enough engagement with them in the first place to ever tackle issues arising. I know there will always be complex issues associated with live events - but in my opinion most of the bad quality I hear is because of bad practices and not unsurmountable technical and acoustic challenges. For this event I want to address those all too frequent short comings, to deliver the best possible sound - and i'm not clear any audio company - even supposed top quality ones - can deliver . So I believe I'm going to have to either do this myself with a few colleagues; or else be very highly prepared in advance as to what spec to request from a high quality company; to outline my expectations and then navigate all of that with them. And hence this preliminary work and inquiry

I'm after something of the highest replay quality - otherwise there is no point to running the event because the spatialised and immersive aspects to what we're thinking about - it will need for absolutely fidelity in replay.
So while I accept the directive to 'seek professional help' - I'm also looking at this juncture for the best technical advice there is to help do something exceptional play back wise to several hundred people; either with a group of people I might assemble myself - or on how to engage an audio company and be able to relay the required quality in crystal clear terms.
My gut instinct tells me to hire the very, very best PA system there is - for far greater wattage needed for the venues in question - and then run them at low levels to avoid any stressing of those systems, all the while ensuring full balanced play back capability from lowest bass to highest treble - and then to look for large periods of setup, testing and rehearsal (perhaps even as much as several days testing in each performance instance) and then to have someone on the desk who is hugely familiar with the program over multiple months.
Thanks,
Kevin.