If you need a multi-timbral synth alternative to TTS-1, a couple of good options are the samplers Native Instruments Kontakt and IK Multimedia Sampletank. Spectrasonics Omnisphere 2 is 8-channel multi-timbral. There may be others ...
But Cakewalk Sound Center, Studio Instruments, Dimension, Rapture, etc. all require a separate instance of the virtual instrument for each sound or MIDI channel. Many other virtual synths work this way, unless otherwise noted. Technically you could use the multiple oscillators within some of the as separate sounds, but it's not the same.
The reason that TTS-1 is still around is probably it's muti-timbral ability and
the fact that it's the only GM (General MIDI) synth bundled with Sonar (edit: thanks scook). Certainly not the best sounds, but works OK to fill in parts with until you can replace them with something better.
Cakewalk Synthesizers, From Presets to Power User, second edition, by Simon Cann:
What Is TTS-1?Cakewalk TTS-1 is a GM2 (General MIDI 2)–compatible, multi-timbral, multi-output,
software synthesizer with 256 preset sounds and nine drum kits. It comes with SONAR
version 5. Let me explain a couple terms in this paragraph:
•
A multi-timbral instrument is capable of playing more than one sound simultaneously.
With TTS-1 you can have up to 16 simultaneous parts. The advantage of multi-timbral
instruments is that you only need one instance of the synthesizer to create several parts.
This results in a lower CPU overhead. (Usually each instance of a synthesizer will take
up some computer horsepower.) In SONAR you can select each of the 16 parts (they
correspond to the 16 MIDI channels) with a separate MIDI track. The maximum number
of notes you can play simultaneously is 128 (subject to the power of your computer).
•
A multi-output instrument has several outputs that can be routed to different destinations.
The advantage of multiple outputs is that you can keep your parts separate when
you are using TTS-1 in multi-timbral mode. So, for instance, you could send a bass
guitar to one output, drums to another, and an organ to a third output.
Why Use TTS-1?If you’re looking for a synth to replay General MIDI files, then TTS-1 is a no-brainer. There
are many other reasons to use TTS-1, such as its wide range of highly usable sounds coupled
with good FX.
Many people use it as a first-call synth, so when they start working on a part,they select a sound from TTS-1 because it has such a wide range of usable sounds. The TTS-1can then be replaced later with another (perhaps more flexible) synthesizer. However, often
you will find that the TTS-1 sounds as good as, or even better than, the sound you try as a
replacement.