Most bass cabinets available are really designed for bass guitar and are voiced accordingly (they are anything but flat).
Also there is a price to be paid for ruler flat response to around 40 Hz: Either exceptionally low efficiency or very large and heavy. There's really no way around this. I have owned cabinets that were truly flat and very full range, however I did not find them to be very practical in actual use.
I am currently using Euphonic Audio cabinets, and I think they have the right idea. If you ever get a chance to try a classic EA VL-208, I think you might like it. Comes very close to your ideal speaker cabinet description.
Also there is a price to be paid for ruler flat response to around 40 Hz: Either exceptionally low efficiency or very large and heavy. There's really no way around this. I have owned cabinets that were truly flat and very full range, however I did not find them to be very practical in actual use.
I am currently using Euphonic Audio cabinets, and I think they have the right idea. If you ever get a chance to try a classic EA VL-208, I think you might like it. Comes very close to your ideal speaker cabinet description.