The models from Heir I have the most experience with are the 8.A and Tzar 350, and those are about as polar opposite as two earphones get (okay maybe the Cardas Ear Mirrors and the Tzars are more opposites). From what I gather though, most of the models aside from the Tzars could be characterized as a bit warm and laid back to varying degrees with the 4.A coming closest to the UERM but with a hint more of that aforementioned coloration. Generally I'm not a fan of overly warm and bassy signatures, but the 8.A is within the threshold of acceptability as far as the overall balance goes, reminding me of the earlier Audez'e LCD-2s. It's a signature I can best describe as comfortable. Like a worn sofa you sink into, though not so much that you suffocate. Pleasant. No major sins of commission. The biggest flaw for me is a certain lack of vital energy in the mids which gives them a kind of dull sense of presence. Vocals on the 8.A are a bit boring as a result. Which I felt was a bit unfortunate, as in other respects they sound fairly natural. Apparently the Noble 8C (<---looks like a frowny face) addresses this exact issue with a boost in that region.
For me, the main attraction of Heir's designs (again, the fringe Tzars excepted) is two fold. First in their maintaining a sense of clarity despite that added warmth in varying degrees. The second is driver integration; say what you will about Dr.Moulton, but I think he can really massage drivers into tandems and is usually very deliberate in his tuning. Those CIEMs I've heard (6.A and 8.A) sound very coherent, natural, and effortless to me as a result. They're not perfect mind you: the 6.A has some upper mids stuff going on I don't exactly care for personally, and the 8.A is a little dynamically 'flat' as I mentioned above. That's why I'm interested in hearing the changes in the Noble line.