Nothing can truly replace leveling presets live, with the band, using your ears. But if you need to get things close, and you can't open it up to full volume to test levels using a Db meter, this approach works pretty well.
You'll need a pink noise generator and a loudness meter application. I'm on OS X and I use Noisy for pink noise generation and Orban Loudness Meter for measuring. Both will work using the Axe-Fx II's USB in and out on OS X 10.9.x. They also both happen to be free which is nice.
WARNING: Disconnect your Axe-Fx II from any output equipment. This includes headphones, powered monitors, power amps, etc. You don't want to hear the sound that's being run through the unit while you're doing this. It's not pleasant and could potentially damage your hearing our your amplification equipment if you accidentally amplify the sound while you're leveling this way.
With both programs installed, fire up your Axe-Fx II and set it up as such:
- The Output Level 1 knob should be set to 9 o'clock
- The input source should be set to USB
- The input 1 gain should be set to whatever you usually set it to for your main guitar
Set your audio output source on your Mac to the Axe-Fx II -- Noisy just sends white or pink noise to whatever source you have selected for your main audio output source. Fire up Orban and set the input source to monitor to Axe-Fx II in the settings tab.
Before we start leveling presets we need to make sure we're running Noisy at a consistent level. To do that, put the Axe-Fx II in to bypass mode by pressing the BYPASS button on the front of the unit.
Press the Pink button in Noisy to start a pink noise stream. Press the Power Button on the bottom of the Orban Loudness Meter screen (it's on the Meters page) to start monitoring. Adjust the slider in Noisy until the ITU BS.1770 Short Term & Integrated meter in Orban is showing you -25.0 LKFS. That's your reference level to which you'll be leveling all your presets. If you don't set Noisy's reference level every time, you'll be leveling to different input values and you'll have to relevel all your presets every time you level. I use -28.0 because that's about what I get when I run my hottest, humbucker-equipped guitar into the front input, bypass the grid, and meter with Orban. So the pink noise is about the same level as that guitar.
To level a preset using this approach, select the preset on the Axe-Fx II and then un-bypass the unit by making sure the BYPASS button on the front of it isn't lit up green. With the preset un-bypassed, run pink noise from Noisy through the patch and set the levels on your AMP blocks until Orban is showing -17.0 LKFS on the ITU BS.1770 Short Term & Integrated meter. Repeat that for any scenes that aren't intended to boost the volume of the patch.
For patches that are heavily compressed, like patches that use lots of distortion or a very heavy compressor block, I like to target -17.0 LKFS because it leaves me enough headroom to use a +6 dB boost Filter block in my presets for solo volume boosts at the touch of an IA on my MFC-101.
For patches that have a lot of dynamics in them, patches that are largely clean amps with no compression, I like to target something lower -- around -22 LKFS. This gives the headroom you need for all that dynamic range. If you don't level your clean sounds lower, when you play, you'll find their peak volume is much higher than a distorted preset or setting in the patch.
Repeat the above steps for any scenes and presets in your unit you want leveled.
Double check things with your guitar at stage volumes. I find I'm doing only minimal tweaking for Fletcher/Munson effect at stage volume when I level using this approach now and it's making patch changes, pre-rehearsal, far less interruptive to rehearsal to level in situ when I get together with the band.