I think others might say this isn't really a battery/power forum, and maybe refer you to the12volt.com (or other?), but....
Bubbling batteries is either from overcharging (above 14.4V) or overloading.
EG - the older a battery gets, the more it bubbles during cranking.
I presume your battery was old? (Though mine last 8 years, apparently most get 2-4 years out of them. I used to consider 3 years life ok.)
If so, new battery. Only 2 things you need to check:
1: - the alternator is not overcharging nor undercharging.
This means long term, not above 14.4V (at the battery - not the ignition or dash supply etc!), and generally above 13.6V (ideal long-term is 13.8V for a fully charge battery).
This does not include short periods above 14.4V just after cranking, or dips to battery voltage (12.7V or less) when low RPM with lights & loads etc.
2: - the discharge rate of the battery with your amp is acceptable (when not charging).
The higher the discharge rate, the less lifetime the battery has (meaning years of life - not only "reserve" time).
And that it does not get flattened. This means typically less than 70% capacity for typical car/cranking batteries, and under 30% or less capacity for deep-cycle batteries (though many reckon never less than 50% for the latter... it's the same thing - the deeper the discharge, the shorter the lifespan).
The final consideration is whether your system is capable of powering the amp whilst charging.
When the battery drops below ~13.6-14.4V, then the alternator isn't adequate. That's not too bad if the battery voltage is still above (say) 12.8V or if the engine is idling or has intermittent high loads (stop, wipers, etc).
The voltage drop is often fixed by upgrading power (and ground!!) wires - often called "the big 3", ad this is ESSENTIAL for any added high-power system. The engine (meaning alternator) to body/chassis grounding wire(s) must be solid. (Often TWO lots of the heaviest wire/cable being used for redundancy, and to halve that voltage drop cheaply.)
Otherwise a bigger alternator is needed.
The crap below is only if you want the common "ultimate" set up.
If you only want the battery answer, desist!
My recommendation is either your (upgraded) system handles it, or - especially if you want an audio battery independent of your cranking battery - you add a second battery next to your amp.
Apart from independence, it can mean a smaller cable can be used from the alternator/main-battery to the amp (since the amp battery looks after the thumps).
And it overcomes the need for a capacitor (which is a very expensive way of providing a much shorter energy reserve than a battery - even $35 AGM battery).
For a second battery, keep the main battery standard (changing this is probably wasted, and using AGM also wasted and may not last as long).
Get a suitable AGM battery for the amp (deep-cycle if wanting to listen with engine off; cranking type for better anti-thump response).
Fit adjacent to the amp.
Run an interconnecting cable between your 2 batteries (or alt & amp battery but that's usually difficult!) with a fuse or circuit breaker at EACH end (as near each battery as possible).
And add an isolator on between.
If you have a charge lamp, I recommend a suitable relay controlled from the charge lamp circuit. Otherwise a voltage switched relay - eg, battery isolator.
If the charge-lamp or battery-isolator relay isn't big enough for your load, use it instead to switch another relay that is.
You could (should?) also add a low-voltage cutout for the amp battery (unless you monitor its voltage or don't flatten it) which - like the isolating circuit - can be a cheap battery protector (like the MW728) that switches another heavy relay - though being voltage controlled, there are timing and hysteresis issues unless a latching circuit and reset are used.