Interesting. I’d do the NDX2 and then the pre ASAP to get best out of the NDX2. And yes you’ll hear the DR on the 300 when the time is right but feed the existing amp first.
The problem with moving the 555PS to the NDX2 is that the bare 272 as a preamp will be way out of its depth in a system of that calibre. Source first and all that, but in this case I’m not so sure. Still, you can try it either way and decide which compromise is best until you can upgrade the pre.
I would either wait until you have the finances to trade in the 272 in for a NDX2, SCDR and 252, preferably exdem or preloved units. Or grab those that come up first at discounted exdem or preloved prices. You can wait to DR your 300 or trade it in against an exdem or preloved 300DR in due course.
This is an insoluble di(tri?)lemma, and is usually sorted by pragmatic purchasing IMV.
@Stu299 you have an excellant system now, and my system is 252/300DR which is now my end system
I started with NDX in 2014 with 202/200 and built over a few years
I sourced a used ND555 last year and did exactly what OP is suggesting, using the 555PS into the streamer and the bare 272 as pre-amp. It lasted less than a week; so out of its depth was the 272 that a used 252 + SC replaced it asap. Everything is relative, but the 272 sounded awful with a decent source. As a standalone pre-amp + streamer with the 555PS it had been superb.
I think buying an NDX2 first would work fine, I just think you would need to keep the 555 on the 272. Then you may well get better results using a digital connection, with the PSU upgrade still benefitting the DAC as well as the preamp.
Then put the 555 on the NDX2 and use its analogue out when you’ve got the 282 or 252.