Some thoughts and clarifications about SLA batteries to consider.
Sealed Lead Acid batteries typically come in two distinct "types"
AGM (Absorbent Glass Mat), which utilise a fiberglass mat laid between the plates to act as a wick and maintain constant electrolyte contact with the plates. AGM batteries are better for high current short duration discharges and are commonly used in standby applications (UPS, Security alarms etc)
They are normally cheaper than Gel
Gel batteries mix the electrolyte with a Silica Gel to suspend the electrolyte throughout the battery maintaining contact with the plates. Gel batteries are normally better suited to lower current longer discharge applications, when correctly specified they suit Deep Cycle applications like Solar or electric vehicles etc.
The Duracell documentation by the OP refers to both technologies in the same low cost battery which seems contradictory?
The Dura12-9F2 listed as a 12 volt 9ah (Amp Hour) Battery with F2 Spade terminal configuration. Capacity is at the 20 Hour rate (C20)
9ah at 20hrs, contrary to apparent public opinion does NOT mean it will run 9 amps for 20 hours, or 9 amps for 1 hour or in fact 1 amp for 9 hours.
It means it will deliver a total of 9 amps over a 20 hour period to an end voltage of ?? (this end voltage should be specified but is most likely to be 1.8Volts Per Cell), which is 450 ma per hour.
Simple maths tells us that 450ma x 12vdc (nominal voltage) = 5.4 watts for 20 hours.
Without discharge curves from the manufacturer it is difficult to assess actual runtime, however looking at similar General Purpose AGM SLA batteries would suggest approximately 1 hour at 20w per channel (without considering any other losses) and 3 hours at 10w per channel.
Cycle life, Depth of discharge/end voltage and Design life.
Cycle life is simply defined as the number of cycles to a specified end voltage, at a defined temperature, with a defined temperature controlled recharge rate. The reason Duracell list such a large range is possibly because they have not defined any of these parameters.
Depth of Discharge is a little more complex, 100% DoD is NOT zero VPC it is when the battery has discharged to a design end voltage.
As an example small UPS's typically discharge to 1.70VPC (10.2 volts at 12 volt nominal) but may go as low as 1.6VPC
End voltage is the voltage that the system should disconnect from the load to either prevent damage to the load or the battery.
Design life is more relevant to larger batteries and systems and again is tied to the operating parameters and duty cycle of the system.
The Duracell battery selected by the OP is designated as General Purpose and will run the sound system but won't last too many seasons.
My strong advice to any SLA battery user is to use a battery charger designed for SLA's, they are very voltage sensitive, leave the battery on float charge at about 2.25 - 2.6VPC @ no more than 25 degrees Celsius
This link is for a battery company that supplies more information regarding the battery design, this is the sort of information needed to design a commercial battery system.