I use the TSS-1 a lot but it is DX only. My worry is that some time in future it may be dropped.
 
All I find is megabyte libraries with wonderful natural-sounding instruments but which then need compression, EQing, gating (esp. drums) and all manner of processing before they sit properly in a mix.
 
I am looking for a lightweight ROMpler VSTi equivalent with bread-and-butter, pre-produced, ready-to-use sounds.

http://www.roland.ca/products/sound_canvas_va/
(Supposed to be a dead ringer for the hardware sound canvas.)

Brando's suggestion is ideal, but I think for what it is, the price is far too high. 

Forte Coyote, also a DXi, is provided with BiaB.  I like TTS better.  SampleTank has OmniSynth, which sounds really good and responds to PC messages ...
 
http://cgi.ikmultimedia.c...ic.php?f=12&t=2953
 
SampleTank takes longer to load up, but has a better set of sounds, IMO.

Sir William
Brando's suggestion is ideal, but I think for what it is, the price is far too high. 

I've heard that opinion (kvr) but at $125 for the standard against which everything else in the category is compared i think it's hardly overpriced. Guessing it'll discount around $99

M1 and Nexus 2...

Brando
http://www.roland.ca/products/sound_canvas_va/
(Supposed to be a dead ringer for the hardware sound canvas.)

Found this:
 
Due to host application specifications, system exclusive messages cannot be sent from tracks to SOUND Canvas VA in the following hosts.
 
[Windows]
Windows® 7 SP1/Windows® 8/Windows® 8.1/Windows® 10
- Cakewalk SONAR PLATINUM/Professional/Artist(32-bit Edition/64-bit Edition)
- Ableton Live9 (32-bit Edition/64-bit Edition)
- Ableton Live9 Lite (32-bit Edition)
- Presonus Studio One 3 Professional (32-bit Edition/64-bit Edition)
 
 
Due to host application specifications, you cannot use control changes to make correct bank selections and program changes in the following host application.
[Windows]
Windows® 7 SP1/Windows® 8/Windows® 8.1/Windows® 10
- Cakewalk SONAR PLATINUM/Professional/Artist(32-bit Edition/64-bit Edition)

Some options (In roughly ascending price order) :
 
- Free GM soundfont with a free player like TX16WX.
- Dim Pro (You should already own if you have Sonar). 
- Dim Pro or TX16WX with Proteus 2000 soundbank (or equivalent, from DigitalSoundFactory)  $40
- Sampletank 2 (Can be bought secondhand at KVR marketplace for maybe $50).
- Korg M1 Legacy $50
- Roland Sound Canvas VA  $125
- Halion Sonic 2  $250
 
Of course there are others, but hopefully these give you an idea of the breadth of Rompler-type virtual instruments out there.

I just downloaded the Roland Sound Canvas VA to test it against my Sound Canvas 8850. It is close but its not the same or as good. It is better than the TTS-1. If you don't have a real Sound Canvas and need one it will do. However I'm sure you can pick up one used for a song. 
 
Of course I paid over $900 for my Sound Canvas. 

$900 ?