How long will an 6000 mAh battery pack power a 12v 60W amp for?
I’ve been playing the speakers fairly loud off of this charged pack
  and it doesn’t seem to be losing so fast. I think I’ve already
  listened 6+ hours on just this charge


Your amp can output 60 watts but if you are not dumping 60 watts into your speakers it won't be taking 60 watts from your battery. Even on full volume, music has an average power level (compared to peaks) that is about 10 to 15 dB down so, if your peak is 60 watts (a big slice of kick drum) then your average power might only be about 4 watts.

Given that your amp may be 50% power efficient the input power into your amp may be, on average, about 8 watts.



In the example above the peak is 2 watts yet the average is only 63 mW so. if your peak is 60 watts, the average will be about 2 watts. However compressed music like hip-hop or drum and bass will be more like 12 dB in terms of crest factor (\$\dfrac{P_{PK}}{P_{AVG}}\$).

If you happen to be listening to classical music it might be an even bigger ratio. Are you listening to classical music on a battery powered hifi? I could make a guess of course!