The cheapest option is just to turn the gain up. It sustains longer, the only problem is that it sounds too..... well, gainy.
That didn't really help did it.
I haven't tried this with mine at low volumes but last night during practice I cranked the mids (to 3 o clock). My guitar started feeding back like some sort of beast the moment I stopped playing but man! It was all fluid and had heaps of sustain. It's kinda the opposite of what I expected. I read that the strings feel stiffer and less compressed when the mids are high.
It's worth a shot, the worst that can happen is that you don't like it, it's free to turn a dial.
Hopefully that will help.