Sustain is a physical phenomenon more than a modeling thing - it's the interaction between the nut, string, bridge, vibrato, and guitar body. It all depends on how that whole system resonates together, and is different from individual guitar to individual guitar (for example, compare any two Strats and you'll find slightly different sustain times).
Having said that, although modeling can't exactly increase sustain, I believe Line 6 have some kind of amplitude curve shaping thingamabob in their modeling, as some guitar models actually create a sharper attack and quicker decay - - thus, less sustain than the physical instrument (for the tradeoff of more punch). You can experiment with different guitar models to find the one that has the most sustain (that is, the full sustain the physical instrument is capable of).
Either way, if you switch to the magnetic pickups, you'll hear the "100%" mark, the total amount of sustain that your personal JTV-69's physical parts can achieve. If it's not enough, you'll need to take generic "any electric guitar" approaches to trying to get more sustain, because there's nothing Variax-specific about that physical aspect. For example,
- Convert the guitar to a hardtail
- Alter the setup (i.e. sometimes higher action is more sustain)
- Experiment with different string gauges
- Change the neck, body, vibrato, nut, etc...
--- Quick edit: Of course you'll also hear the physical sustain unplugged, too. Also, something to watch out for is to make sure the magnetic pickups aren't too close to the strings, because their magnetic fields attract the metal strings, causing "drag" that reduces sustain.
Edited by mdmayfield, 20 August 2014 - 07:14 AM.