Wow, that's a very detailed write-up.
Perhaps I'll create a Boombox sticky and add the link to your write-up.
Based on the number and length of these boombox threads, I really think there should a separate forum for just boomboxes.
41Hz could become the defacto source of all that is boomboxes & increase sales or the AMP6 too!!
One work of advice though, do not let the VRLA become fully discharged. If it does become quite discharged, then make sure you recharge it right away. These batteries suffer from sulfation if left in a discharged state for any length of time. Also you will increase the number of charging cycles over the life of the battery, the less you discharge it.
If you only discharge it to 60% instead of say 40% you will extend the life of this battery greatly.
Then again, if the cost of replacing the battery every two or 3 years of constant use is not a concern, then this will not be an issue for you.