Assuming you will not have your own separate monitor and you will be DI'd to the PA I recommend in the 500-750 price range maybe the GenzBenz Shuttle 3.0 or 6.0, MarkBass LittleMark, or other similar lightweight, lower priced head.
If you do not have a monitor you will need a cab. Lots of portable, good quality 2x10's out there that should fit in your price range after the head.
OR...if you are provided a monitor or headphone mix get a Tech21 VT Bass Deluxe, Radial BassBone, SansAmp, or older Line6 XT or X3 and call it a day.
+1
Good advice, plus there is plenty of other good advice above, too. If you really want a rack system, shop around. A lot of geezers are ditching their lead sleds for light weight switching amps (not me).
A USED 1200+ watt QSC or Crest power amp ought to go for under $600 used and shipped. Pre-amps can cost anything, but you can find a BBE BMAX or Sansamp RPM or RBI for under $300. A used 3 or 4 space rack ought to run you under $75.
So call it a $1000 rig with no cabs, if you go racked power/pre. A used Shuttle 6.0 (600 watts) will go for under $500.
Really, any good 500+ watt head and good quality 210 will do the job. Read more threads, visit music stores, listen to live bassists, and save up. Visit TB classifieds and Craigslist.
Despite the orchestra, your application is really no different from many others, in that you will be heard through the PA in a medium-large auditorium, and your monitor, and probably the conductor's and ensemble's monitor for the electric bass, will be your amp.
Are we to assume there is no double bass section in your orchestra?
