I think that descriptions like "dark and warm tone" are a bit ambiguous and probably a bit subjective. The first question that should probably be asked is: How much do you plan to spend on your own 7 string classical guitar? If you can spend $2,500/$5,000 or more, then there will be many luthiers who will work with you to produce an instrument that meets your needs. However, if your price range is $499/$1000, then you won't be able to find a luthier who will build you an instrument; you'll have to find one that is mass produced, and there aren't many choices.
There are only a few companies that I'm aware of that currently mass produce 7 string classical guitars. Ibanez makes a 7 string with a cutaway, which has a sold spruce top, laminated mahogany back and sides, and a rosewood fret-board. This is a nice entry-level seven string classical guitar, but I can't say that I'd describe the tone as "dark and warm". Try one out and see what you think.
Rondo Music makes a budget 7 string classical with fanned frets for about $499, but you won't typically find them in retail stores, so you probably won't have an opportunity to audition one to see if you like the tone. Plus, IMO, a fan-fretted guitar that isn't produced to the scale of the original Brahms guitar, thus allowing a high A string, is of dubious value. The whole point, IMO, of the fan-fretted instrument with a slanted bridge is to allow one to extend the guitar's range in BOTH the bass and treble areas. I've informed Rondo Music about this and we'll see if they bite and begin to offer a budget Brahms.
Giannini makes a seven string classical guitar for about $499, but it's from the budget series of Giannini guitars and I've been warned to steer clear of these. I can't say whether these warnings are warranted or not.
I personally own a Giannini Asturias, which retails for over $1,000, but I was able to get one on e - b a y for about $750 (can't remember the exact price). You can see another iteration of my guitar, here:
https://reverb.com/item/2609380-giannin ... zil-gnc4-7
I'm very happy with my Asturias seven string. It has a solid red cedar top, solid mahogany back and sides, and an ebony fret-board. I think that "dark and warm" fits as a description of the tone, at least to my ear, but I can't say whether you would also hear it that way. Having that extra bass string makes many pieces a whole lot easier on my old suffering hands.
The nice thing about my Asturias is that it's constructed in a way that will make converting it to an eight string rather easy. Drill a new whole in the head-stock, replace the nut and bridge, and I'll have an eight string, albeit with a somewhat narrow fingerboard for finger-style.