FAD is a pretty confusing and esoteric company, and this is reflected in their product lineup.
Here's what this line-up looks like as of now:
Piano Forte II
Piano Forte VIII
Piano Forte IX
Piano Forte X
A couple of things going on there. First, I put the upcoming Heaven II in both categories because I've heard conflicting things from different sources about whether it's going to be a BA as the name would suggest or dynamic. You'll notice that the list is kind of a mess right now, and that's because Final seems to be in a period of transition with their product line. Here's a brief history:
At first they only made dynamics, and the designations for there were all four digit numbers (eg. the "1601" of the FI-DC1601). Within each line, the sub-designations which indicated their ranking was a bit difficult to decipher. For the 1350, the "M1" and "M2" were straightforward enough, but on the 1601 models they used a system of "SB," "SS," and "SC" which are derived from the materials used in their construction---brass, stainless, and copper---but lack a logical connotation. Then they brought out their line of BAs which they dubbed "Heaven" and had letter designations (eg. "Heaven S" aka FI-BA-SB).
When Final decides to phase out a particular model or series, they'll usually keep the older model active for a while and bring out the replacement alongside of it. For a brief period they coexist, before they eventually get rid of the old one. It makes things pretty confusing. For instance the Heaven A was replaced by the Heaven C, but briefly both existed side-by-side despite being very similar. Same with the 1601 and 1602 series.
The older dynamic models were eventually phased out and replaced: the 1350 with the Adagio lineup and the 1601 with the 1602 aka Piano Forte lineup. The use of the Piano Forte name for their flagship dynamics was a bit confusing because it was also the name given to their cheapest product, a pair of entry level earbuds. However there are similarities in the design, particularly their fixed tipless nozzle and the construction which mimics horn speakers. Anyway, in both cases these new model lines favor much more straight forward numeric designations to determine their ranking: II, III, and V for the Adagio and VIII, IX, X for the Piano Forte line.
Most recently, they've started replacing the models in the Heaven lineup while still retaining the Heaven designation. The IV and VI currently coexist with the C and S, but I'm fairly sure the latter two will eventually be retired. So just in terms of numerical designations one has:
Piano Forte II / Adagio II / Heaven II
Piano Forte VIII
Piano Forte IX
Piano Forte X
The question marks indicate that there is model with a numerical value of seven. If I had to guess, I think maybe the eventual successor to the FI-BA-SS (which is now actually discontinued) could occupy this position, perhaps as "Heaven VII." Alternatively, they might bring back their older concept for a flagship level dynamic with a plastic housing, something that was originally slated for the 1601s but dropped. Perhaps a higher-end plastic Piano Forte this time around? Again, all just baseless speculation.
As for their fullsized headphones, the fact that the Muramasa VIII was given a numerical designation, coupled with FAD's own precedent, suggests it was originally intended as one in a series of models. Of course, this begs the question: was it the highest end or the lowest in the ill fated Muramasa lineup? Given that it was already 6000 USD or so, the prospect of it being the cheapest in a potential lineup is rather frightening.
FAD seems to be going for a more practical approach these days, something that is reflected not only in their new numerical designation system, but also in the designs of their earphones in general (the new Heavens look quite utilitarian). The new fullsized headphones that will be replacing the Muramasa have already been confirmed as a series of three models. I've been calling them "Pandora" simply because this was one of the names given to their prototype designs shown at the Hong Kong AV show. It also implies a certain mystery, and when FAD was showing them off at RMAF, they covered them in camouflage like some automakers do with their new cars. The top model will be all metal, the bottom model plastic, and the midrange model will be a combination of the two materials. Prices will range from $1000 to $1600 reportedly, a definite nod to practicality compared to the Muramasa.
No word on their numerical designation, but it's safe to assume it will fall within the range you mention.