I don't know that you need to pick the K10 from the Noble line, but I have no complaints with mine. I hope to have it for a good long time, as any other ciems will sound a bit different to me, but probably not better overall. If at all possible, you should really try to audition them before committing to buy, getting ear impressions made, waiting to see if the impressions are good enough, long build time, etc. Noble is great to deal with, though, and you can always contact them through their website, or send a PM here to bangkokkid (Brannon Mason from Noble Audio) for suggestions. The craftsmanship is excellent, and you can take a look at the company Instagram page for design ideas. Mine (see my profile pic) is not a custom Wizard design, and still has the beautiful amboyna burl wood faceplates.
For me, I wanted to get a top of the line ciem, if I was going to bother going through getting one and waiting a long time to receive it. I didn't want to settle for something less based on price, but I understand not wanting to pay that much (or not having the funds). I just prefer the K10, but from the 8 is also excellent and not what I'd consider a big step down. You're really getting into diminishing returns territory here, and for me it's more a matter of preference, as opposed to one model being "better". I also have a universal Noble N5, which is the best universal iem I have ever tried. It doesn't quite have as much body and cohesiveness compared with the K10, but it varies a bit depending on the seal I get. Does the K10 have twice the performance? It's hard to say, but overall I think it does everything just a bit better.
Listening through a ciem and a full-size open headphone are very different experiences. I can't say that the K10 is a good approximation of the Sennheiser HD650 (I have one also), as I find the K10 has a more open presentation, with more separation between instruments, and possibly better bass extension and definition. Often you will see people trying to describe headphones or iems in terms of their presentation of treble, midrange, and bass. In the case of the K10, I think it is amazingly coherent, without any frequency range overpowering another. If a song was recorded in a large, open space, you will head it on the K10. If there is a powerful underlying bass line in a song, the impact is there.
We all hear things differently, though, so other folks may disagree. Depending on a specific recording, sometimes you will get more of the "inside your head" effect, compared with a full size open headphone.
And yes, I definitely get some background noise (most here call it hiss, you called it hum) with the M8, on low gain, if I am using the K10 or another sensitive iem. It's not terrible, but is always there and can intrude during music with quiet passages or sparse instrumentation. The bigger problem for me, though, is that I get very little usable range with the volume knob. Depending on the loudness of a recording, sometimes I barely get the knob past the point where both the right and left channel are matched in volume. That's why I think the Mini M8 will be a better choice for you, if you intend to mostly use it with a ciem.
