@mrkim --
I'm unfamiliar with Mainstage, but from the discussion, I'm guessing you might find Unify to be a useful Windows platform for loading and mixing various VSTs in realtime, without the overhead of a full DAW.
The app itself is here:
https://www.pluginguru.com/products/unify-standard/It was just released at last week's NAMM, but I've been using early versions of it for a few weeks and I'm very impressed with its stability and power. It recognizes nearly all the VSTs on my system, and has a simple but very flexible interface. There are already several instructional videos, and the third-party reviews are starting to appear.
John "Skippy" Lemkuhl is the project creator, with extremely capable programming by an experienced MIDI / audio architect named Dale, and they are both very responsible and dedicated creators. Skippy's background includes patch design for many generations of Korg and Yamaha synths, along with innumerable collaborations with recording artists in a variety of genres. I've been working with Skippy's sound design products since the 1990's (he created a lot of the patches in the Korg Wavestation, and now the new Korg WaveState).
I have no connection to Skippy other than admiration and long experience with his work. I am now using Unify regularly as a quick, ad hoc VST host for all kinds of things. There are several soft-synths that I use for sound design, often without any need for my DAW -- I just want to start playing and sculpting.
The present release doesn't write audio files directly, so you would have to run the computer output through another app like Audacity to capture the sound into a file -- or, of course, just load Unify into a DAW if you're planning to keep what you're playing.
I hope this helps,
Allen
