Should I wait until I can afford the X9000 plus some not-too-expensive amplifier (might take a while...) or buy a cheaper e-stat?
This belongs to the X9000 or the Stax thread... so it's off-topic here. In my experience, the better headphones get, the harder is to find their best environment, partly because they expose far more weaknesses of upstream than lesser transducers. I have tried many amps with the X9K until one "clicked in", then I "maxed out" (using the best parts available) with that design, and then the experience was transformative... not fallen apart at all, but more of a single holistic organic experience, close to good acoustic
concert experiences - OK, still retaining the X9K character, but in a better way. The X9K is not a requirement, people I know had similar experiences with the 007 Mk1 and the SR-Omega, clearly preferring them over the X9K. It might also depend on your preferred musical genres... About expensive amps: a friend achieved very good results (he says best results) with the X9K using a modded SRM-252 + a special LPSU (check the X9K thread). A Lambda Sig or LE or SR-Lambda (normal bias) can also sound excellent, and the total cost of say a modded Stax T1 with e.g. a Lambda Sig is affordable and comparable to the IO-12 on price, but much better on sound. Read the Stax thread, you'll get good tips. My pref is with the X9K among the Stax, given a well matching amplification. Nevertheless, I like even more my two Phenomenon e-stats, and best I like my good speaker setups. IMHO the best value in wired (closed) headphones today are the new issue (post 2022) Denon D7200's, and the best wireless are the IO-12, for me.
IMHO the IO-12 is reproducing only a small part of that "best" experience and it sounds more rough, not as fluid and deep and rich - but it reminds to it... and has the convenience and good-enough'ness for many use cases, so it represents an awesome value for me - not so much with the stock ear pads, but YMMV.