Too late - you purchased the Yellow Top!
You can read what others think about using Yellows as crankers. (IE, you may find Reds are suggested, or that Optimas are not.)
IMO for an audio setup, it's best keeping a wet-cell as the cranker and fitting a 2nd battery near the amp. That negates the use for a capacitor and - with an isolator - allows the audio to run with the engine off without effecting the main/cranking battery.
For SPL, even though floodeds (wet-cells) are usually the best, some use AGMs to avoid the need for venting. (Local die-hards add vented enclosures.)
Note that Optimas are AGM batteries (Absorbent Glass Mat, aka VRLA = Valve Recombination Lead Acid) and hence - under most applicable vehicle etc Regulations - do not require venting. (IE - they do not have to be enclosed on a sealed vented (... to the outside...) enclosure.
Hence you should not need to worry about extra venting. (IMO an AGM is more likely to melt or explode than cause external hydrogen ignition.)
Dual-battery set ups should have an isolator.
If you have a charge-light circuit - ie, a alternator (regulator) with a D+ or L terminal - IMO the best isolator is the UIBI - a normal relay that is triggered by the D+/L circuit.
Otherwise a voltage-sensing or aka "smart" isolator.
The isolator (relay) is inserted in the
normal twin-fused battery interconnection - ie, batt#1 - fuse - isolator - fuse - batt#2.
Note that the above isolators are automatic. The UIBI only connects when the alternator is charging. Voltage sensing isolators are supposed to behave the same, but in practice have turn on delays, and often turn off delays (namely till the battery voltage drops below a certain level - eg, 12.5V, 12.7V. 13.2V. 13.5V etc depending on model).
You can also use manual switching - eg, a relay controlled by IGN+12V or ACC or a switch. But if you leave IGN or ACC on, or forget to turn off the switch....
Also IGN+12V means paralleled batteries during cranking. Whilst sharing cranking current may be good, the interlink relays and fuses and cable have to handle it. And interconnecting a full to a flat battery (when not being charged) can have its problems or hazards.
Note that the UIBI is just a manually controlled relay but with its coil connected to D+/L instead of ACC or IGN or a switch.
Note too that not all alternators (D+/L) may be able be able to energise all relays.
In such cases, a transistor or MOSFET or a smaller relay can be used to
buffer the isolating relay.
The same principle is used to increase the current capability of the isolator - eg, if you want a 200A isolator but only have an 80A (voltage-controlled) isolator, use the
isolator to energise a 200A or 300A relay etc. (That's much cheaper than buying a 200A smart isolator etc.)
I don't know the power demand of your audio system.
The above is for big amps in the boot, er... trunks, and would be pointless for HU amps or internal amps - unless wanting to run for extended times without the alternator.
The only addition for BIG audio systems of ~3kW etc would be a cap to protect the AGM if extra AGMs cannot be added.
That reflects that AGMs do not like supplying big current. Though they can, it does damage them. (And I'm talking current peaks etc - not "
within spec for a given lifetime" currents.)
And the above reflects why wet-cells should be used as the main cranking battery.
I hope that helps. I've missed certain basics as well as detail, but a search should find more (eg, google "uibi" or "uibi oldspark" for what UIBI means and why, adding manual controls & low-voltage cutouts etc).