I'm in the process of finally getting all my instruments on their own outputs, so I can EQ them separately. (Quite a task, as I write for orchestra). Now, until a moment ago I had the workflow as follows: track outputs to sound card, with a send to reverb bus, which outputs to mix bus, which outputs to sound card. I just figured out that the track outputs should be outputting to mix, so I changed them. That made the volume way too loud. Where am I going wrong?

Depends.
 
What plugins are on your master buss?
Is your Master Buss at 0dB?
Is your Master Buss the ONLY THING that points to your Soundcard?
 
Could be you were just hearing a reduced volume before you started to sort out your bussing structure

Here's a little trick Daveny taught me.
 
"SELECT ALL"
Go to "TRACKS"
Property's / Output and here you can assign both MIDI and audio outputs. Assign all to "MASTER"
Now everything is going to the correct place.
 
If you want to add Sub mix busses then you do the same but highlight only the tracks you want to group and then assign them to that buss instead. There is no point to make a sub mix bus for a signal instrument unless you like this on hand in Consul view. 
 
Now make sure your EFX busses and Sub mix Busses are also pointed at the MASTER, not the sound card.
ONLY the MASTER should be pointed at the sound card. All else is routed to the Master first.
 
Then in your case your using soft synth's so you'll have to add the efxs sends to the soft synth Audio track, This is tricky if its something like the TTS_1 as you'll globally be sending the whole shebang to the EFX bus. 
 
Also note that sending a track to efxs will increase it's volume to some degree.
Sounds like you somehow double up on your sends or busses.
 

When you say Master, is that the same as Mix? Also, on the Soundsonline forum, people are telling me to just use multiple instances of Play, rather than setting up multiple outputs on one or a few instances. Supposed to be easier on CPU. what about that?

Shouldn't happen. When you route to the hardware directly, you're actually going to a hidden unity-gain bus whose output is then sent to the audio interface. A master bus set at unity gain and no effects will merely be a transparent layer in front of the hidden bus. 
 
If you're seeing a volume increase when switching to a mix bus, it's because a) you have duplicate routings, b) the volume or trim/gain on the bus isn't at zero, or c) there is an effect in the bus fx bin that's boosting the level.
 
To check for (a), mute the master bus. Everything should go silent. If not, then you still have some tracks or busses going directly to the hardware. To check for (b), double-click on both the master volume and trim/gain sliders on the bus to zero them. To check for (c), bypass effects in the bus fx bin by right-clicking on it and selecting "bypass fx bin".
 
Also make sure your reverb bus is 100% wet to avoid adding gain there.

Re: using multiple PLAY instances rather than multi-timbral instances, you're getting bad advice. The sample player will use resources more efficiently when one instance handles multiple instruments, versus having a separate instance of the player for each instrument. 
 
However, there are situations where you would want to use multiple instances. For example, if you need to freeze sections to avoid glitches at low latency, using separate instances of the PLAY engine will allow you to freeze instruments independently from one another.
 

Bristol_Jonesey
Depends.
 
What plugins are on your master buss?
Is your Master Buss at 0dB?
Is your Master Buss the ONLY THING that points to your Soundcard?
 
Could be you were just hearing a reduced volume before you started to sort out your bussing structure

I only have Spaces on my master bus (I'm assuming you mean the Mix bus?). On Spaces, I have the wet setting at 100%, and the dry at 0%. Mix bus volume=0. Only mix is routed to sound card.

By Jove, I think I've got it! I deleted all busses and started over. This time I did not put a send on the MIX bus, just set its output to soundcard. That seemed to do it. No volume gain, everything is as it should be. thanks for all the help, couldn't have done it without you, guys.

I guess the confusion here is this "MIX" buss, I just realized that you can take a blank template, insert a bus and call it anything you want. Just so happens I always call it the "master" .