Really wanted to get the AER Compact Mobile, but it is just too expensive, so figured I'd research how to make a compact 12V rechargeable battery pack for multiple uses and just get the Compact 60.

What I need to know is how many volts a 60 watt guitar amp actually uses to do the math for how long of battery amp hours (AH) I need - thinking 3-4 hours if I'm playing in the subway.

This link shows you how to do the basic math to figure it out (they use a TV as example):
http://www.satcure-focus.com/icetech/page11.htm

So far these links below are what you need, just not sure what size battery to get in relation to how much power a 60 watt amp will actually use. So far I'm thinking the 12 amp hour one:

http://store.batteriesamerica.com/universalbatteryub12120.aspx

http://www.donrowe.com/inverters/voltec_400.html