pre amp level is in general not controllable from within a DAW, at least not from Sonar (for that to work, there must be special integration with particular hardware since there is no standards to do this globally). And so Sonar iPad app, even if/when implemented will be unable to do that (except may be for some Tascam interfaces...).
But Sonar can be controlled by:
1) any MackieControl compatible iPad app. You can find many positive reports in this forum about Lemur
https://itunes.apple.com/us/app/lemur/id481290621?ls=1&mt=8 2) any other OSC iPad app, f.e. TouchOSC in case you do not afraid to be the first who is using such combination:
http://www.azslow.com/index.php/topic,295.0.html Many hardware can be controlled by special (Be X32, A&H QU) or mentioned (RME) apps.