because you started this whole thing with two serious Code violations. You thought those things were allowed, and because of that you think these other things should be allowed.
So let's reset and correct the mistakes.
First, that "adapter" you've heard about to turn a NEMA 14-50 into four 20A outlets.... one of two things is true. Either
it is counterfeit foreign junk that is not legal to sell or use here, (and due to Amazon's cunning lawyers, they're a "Free Trade Zone" and you're the importer/smuggler).
The adapter is UL listed... but you have been incorrect to assume "it's just wires inside". Not at all: it's an active PDU, effectively a sub-panel potted in plastic. Internally, each of the receptacles is behind 15A or 20A fuses (and they may be potted and not replaceable, so blowing a fuse "bricks" the adapter. UL is OK with this, in fact it prevents up-fusing).
Second, putting a 20A breaker on a 30A receptacle is a direct and blatant Code violation. Every breaker must match the receptacles on it, with only two exceptions enumerated in NEC 210.21.
The two exceptions are for 40A circuits, since NEMA refuses to define a 40A receptacle (they're running out of viable pin configurations). And for 15A receptacles on 20A circuits since UL requires 15A receptacles be internally rated 20A. This is a "one-off" rule for 15A receptacles only. There is not some principle that makes it scale to other outlet ampacities.
Neither fault should be tolerated. Don't use a NEMA 14-30 and adapter - use dual NEMA 5-20 receptacles wired in MWBC style (feel free to use two receptacles of 2 sockets each). Remember neutral must be pigtailed, you cannot use a receptacle as a splice point for neutral. Leave the breaker 20A.
Now, with those corrected, your ideas fall apart.
Code explicitly allows 15A receptacles on 20A circuits for this reason. The question I'm asking is whether there are code compliant approaches with 30A circuits as well.
Nope, that was a "one-off" specifically for 15A receptacles. That is because UL requires 15A receptacles to be rated for 20A internally. They make no such requirements of any other size. A 20A receptacle is not rated for 30A internally.
I deliberately oversized it because large amplifiers can have high peak loads. The amplifier uses a NEMA 5-20 plug.
Well, you're allowed to do that all day, all night. You can always use larger wire than is mandated if you can fit it on the terminals. However, 20A sockets must have a 20A breaker.
I want to upsize the breaker from 20A to 30A to prevent nuisance trips for peak loads. But I know that i am not allowed to simply put a 20A NEMA 5-20 receptacle on a 30A breaker.
Hence my question: is there a code compliant receptacle solution that doesn't require an adapter?
Nope. No way around it, outside of using a sub panel or PDU, which is what you were doing if you used a UL listed adapter. Any of this defeats your concept, which is to have 30A over-current protection supplying your subwoofers and no 20A protection.
If you really need that, you should talk to your subwoofer supplier about subwoofers that are designed and UL Listed to take a 30A power supply.
Although rather than seeking a wild one-off design like that, for the same wattage, you would be better off seeking European 230V subwoofers designed for their larger 230V/16A circuits.