Lowther or AER or Fostex drivers in appropriate cabinets are the standard choices for low power SET users. If you are getting a 45 amp, then high efficiency is a must. This narrows the choices to Lowther or AER. For $6k, there are numerous very good possibilities, and even a new set of Hedlund back-horns, for around that figure.
I use a 45 amp, and have Lowther EX3 drivers in modified Voigt Pipes, and like the sound very much. Remember, with back loaded horns 90% of the music is direct-radiated, with the horn only coming into play under about 300Hz, to augment the bass response. This makes the driver the key ingredient, and the enclosure only secondary, to suit your taste in bass response. If you're handy, you can make a set of Voigt Pipes in a weekend for very low cost. Not a complicated project. The drivers are only $925/pr. I have a total of $1500 in mine, and they are made of nice oak.
An alternative, would be a set of Lammhorn 1.8 with Lowther drivers that would be well under $6k used, if you can find a pair. These speakers would challenge the best of anything, as long as it remained over about 50Hz. All SET based systems will be challenged under 50Hz, due to the need of higher power to drive deep bass with authority. I don't find this any problem, and enjoy the better coherence that is provided in the more prevalent frequencies.
If you like front-loaded horns, then the Oris 150 with a powered subwoofer, may take you where you want to go. They use Lowther drivers behind circular horns, and have a separate bass driver, cabinet, and amp, to handle the bass stuff. Sort of like a budget Avant Garde system. Sound is good from the reports I've heard, but I prefer back horn systems over front horns.
If you have some particular questions about this, I will accept emails on the subject. I live with a system like you describe every day, and am well informed on this subject.