Hisssssss on mixer main outputs - A&H SD12-2

I've got an old Allen & Heath SD12-2 mixer that has quite a bit of hiss on the main outputs.  The channel direct outs sound good, but the main outs have a noise floor that's pretty high, regardless of how many channels are being routed there or the level of those channels.

Here's the main output schematic:


I've already replaced all the caps.  I'm thinking a transistor swap is the next course of action.  Based on the datasheets, the ZTX109B NPNs look to be the same as BC109Bs.  Would I be better off with a different transistor?  What about that lone 2SA562 PNP?

Well besides the obvious problem of spelling bus wrong (only one 's' please), yes there is much room for improvement by replacing that two transistor gain stage with something more modern (not just noise). I would be very tempted to cut in a modern opamp, since not only devices but resistor values, and general approach will limit what you can get from tweaking on that same topology.

If the rest of the mixer is using this level circuitry, you may be fixing only one link of a weak chain, and the overall result may still not live up to the new improved bus amp.

JR 

I can't give advice on the circuit, I simply don't have the know how.  but I do have a SP12-2 and can tell you this. mine is very sensitive to picking up noise from surrounding electronicts...especialy computer mouse and CRT monitors seem to be troublesome...my solution is simply to keep them apart...I do kind of like how it sounds thou...I also have a Mackie, but that one just make me feel indifferent, so the A&H is the one I use..( but oh, how I wish I had a REAL mixer/consol)
j

I squashed a bunch of buzz by rotating the power tx inside. 
I really haven't used it since I bought it, and as such, I'm trying to sell it now.  I have a potential buyer, but I told him I'd see what I could do about the mix bus noise.

As JR suggested, I think a couple of opamps may be the best remedy here.