Makers of JamUp, Positive Grid, are releasing a new guitar app for iOS in a week: BIAS. BIAS is a custom virtual amp maker. You can mess around with a lot of the stuff you could customizing a real amp: changing preamp tubes, switching transformers, setting bias (naturally), exact mic placement in front of cab etc.
When you are done, launch the custom amp in JamUp and play it like any other model. This is truly porn for iOS amp sim users:
BIAS will launch for iPad november 13 (iPhone version a bit later on), read more about the app on Positive Grid’s web site.