I've installed Rapture Session and SI Suite. I wasn't aware of TTS1 until now, so I'll look for that.
 
I guess my question would be: which provides the most realistic-sounding instruments?
 
Also, is TTS-1 GM-compatible? If it's got decent instruments, that sounds like a good compromise between quality and convenience.
 
Operating without an external MIDI module would be a fine thing!
 
Thanks, everyone.

TTS-1 description

Well this had me going until scook confirmed I'm not crazy because I too looked at the versions list saw you were correct. Got me thinking "well maybe you get TTS-1 from CCC" or ?  
I have installed HS on a re built laptop that has not had any other version installed and it has TTS-1. 
 
So this is the route the OP should try first as it's so simple to just OPEN any midi file and it will play exactly the way the composer wishes. To make the "OPEN" dialog automatically insert TTS_1 you need to un check all midi outputs in Preferences midi devices. You can leave the Inputs as is. 
 
I work with downloaded midi files from time to time and this always works for me. 
I then improve the sound by inserting better VST one by one
SI Bass will improve things as example, you will have to transpose +12. 
With Home Studio you can then insert SI Drums etc. 
 
It's as simple as OPEN the midi file
Now insert a few other vst's and simply re assign the output of the midi track to the new sounds. 
Home studio is different than the regular versions and does not offer options when you insert a synth. 
There is also no synth rack. 

shaggydawg
I guess my question would be: which provides the most realistic-sounding instruments?

My vote would be for Rapture Session, which is the sibling of Rapture Pro which offers more options for sound design. Still, Rapture Session has parameters for shaping the sound. SI Drums is also no slouch.
 
I've asked here before if anyone uses TTS-1 in their productions, and no one responded. I've most often seen TTS-1 talked about for "scratch tracks" when composing -- a placeholder of sorts where better sounds are brought in later.

I've downloaded some midi files that seem to optimize GM parameters and sound amazing using just TTS-1. 
But I guess you need to know how to tweak system exclusive etc. to get that quality from GM.
 
TTS-1 is something I  will use in one form or another on almost all my backing tracks and recordings.
If you explore TTS-1 menus you will find a lot of way's to edit sounds, add effects and even split it into 4 parts. Not only that it's been rock solid for me and uses very little CPU. I cannot say the same for many other VST's. 
 
I one is going to use Sonar live you want stability and avoid complicated set ups. I would never return to using outboard sound modules. I do not miss the crazy things that used to happen.  I left that behind in 1993. 
 
Backing tracks are not studio recordings and one should not treat them as such. They will be played though a PA system and for the most part are to be in the background and not a featured performance. I want people to listen to my singing and guitar so I don't add anything fancy to mine. And if I use a download a song I strip away the fluff. 
So even though say, Tru Pianos is a way better sounding VST the TTs-1 piano sounds more than fine in a backing track mix. I find no fault when used that way.  I also use the organs, strings and various instruments when needed, Like horn stabs. 
 
TTS_1 drums can also shine though with the exception of the snare which is pretty old school. But the rest of the kit including the Kick really cut through well. I've had more problems trying to use Addictive drums Kick and ride and I've returned to using TTS-1 for those 2 parts. There is also very good percussion and the Brush snare is the best one I've found to date. 
If it was the only synth I had I would not be heart broken.
 
Rapture has some good stuff but I find 80% of it content useless to me.  
 
One of the great things about HS is you can certainly scrounge around for many excellent free and almost free VST instruments. I added Strike2 drummer to my HS for only $43 when they had a sale. It has replaced Addictive drums for me. I still use Session drummer because I can use my own samples. 
KVR site has many freebies. 
 

Everything Johnny said applies, but I will add that TTS-1 is the only GM compatible multi-timbral soft synth that comes with Sonar. 
 
It is a good starting point for a mix.  Then you can selectively replace sounds that you are not satisfied with by adding other soft synths for select MIDI channels.
 
There is a cool soft synth by AIR Music Tech called Xpand!2 that has thousands of sounds and is 4 part multi-timbral, which makes a good companion for TTS-1, to replace certain sounds with better ones.
 
Xpand!2 lists for $99, but can often be found on sale for $1
 
https://www.audiodeluxe.com/products/virtual-instruments/air-xpand2

Good one John, I was going to mention it but didn't know if it was still on sale. It is huge!!
I haven't had time to try much of it yet but so far it seems on par with Dim Pro. 

Cactus Music
Good one John, I was going to mention it but didn't know if it was still on sale. It is huge!!
I haven't had time to try much of it yet but so far it seems on par with Dim Pro. 

I think that the multi-timbral setup in Xpnad!2 is huge.  The presets are ready to play without a lot of tweaking, and cover most any sound needed for an arrangement.  The sample quality is good, much better than most GM sound sets.
 
I do still like Dim Pro, but it has a special place for me.  It is a sample based synthesizer with lots of samples and with a lot of tweaking potential, but is it not multi-timbral, unless you count the 4 elements (oscillators) in multi-timbral mode, which that can get messy.  As far as bread and butter sounds, it is hit or miss.  But I do like a lot of the sounds that it comes with!