Let me first say i have not heard any Omegas. I’d like to.
I do have a lot of experience with FR diy thou. A LOT.
The stock Fostex you usually see (FExx6) are something that is sort of a taste thing. They have many issues, 2 being that thet really want to be in a proper horn (the factory recommended ones do not qualify, and they want ot be driven by a high Rout
amplifier (tyically SE tube amps, but that scene is changing). Their rough edges can to a great extent with modifications. 2 specific ones are fairly simple, a 3rd requires you to get over fear of taking a scalpel to the dustcap. More serious modifications take some serious practise (something i no longer do but am very happy to help others with) This could be the Fostex you heard, but tweaked up to overcome most of their major issues. Note that it is not even worth considering listening to any Fostex without at least 100 Hhrs on them (the case for many FRs who’s suspension has a much trickier job to do.
The TB W8-1808 is a quite popular driver (not quite as popular as the sister W8-1772). AFAIC overpriced and fairly coloured.
The Mark Audio Alpair 10p is very popular, hard to get, one of my favourites, but also has a quite specific voicing, which includes a warm inviting midrange and a “vintage” top end. A favourite of many with SETs. The sister, metal cone, Alpair 10.3 is probably the more accurate, with a more ectanded top (but sometimes the small amount of very HF ringing may find some lookign for a filter (or A10p).
Both are getting hard to find, the newer mono suspension Alpair 11ms has an edge mid/top, bass largely determined by box.
The Alpairs are my personal favourite but given the large number of compromises that any loudspeaker has, and how much what is in front of it can affect performance, any speaker, FR or not is a very personal choice.
DIY offers a huge range of possibilities that no built “store-bought” box can offer. It also requires more knowledge (or luck) and access to tools to build. Once that last is dealt with a competitive sounding diy box for a fraction of what you would pay even direct. Whether it is as pretty is up to your skill.
DIY also allows you to specifically tailor the end result towards your preferred set of compromises and to best match your room.
I do have a buddy who is workign thru my speaker collection in search of a box that statisfies him. He has been thru at least one set of Omegas, and currently has Alpair 7.3eN in Frugel-Horn Mk3.
It is hard to hear an example of FR diy boxes so the journey often involves multiple builds.
Lots more in my head, i am happy to answer questions.