If it's ok to think "outside the box" and still respect the intent of this post, three things that come to mind are placement, toe-in, and speaker cable. 1) Positioning the speakers closer to a corner will emphasize the bass frequencies and subjectively reduce the relative loudness of any treble peaks. This is a compromise because some of the "magic" of a single driver's soundstage may be lost in the process. 2) Spacing the enclosures as far apart as possible without losing the center image, and pointing them straight ahead with no toe-in, will increase the listening angle and reduce the treble intensity at the listening position. 3) SET amps are especially sensitive to speaker impedance, and many single driver enthusiasts use very small gauge speaker cable and internal wiring to increase the total resistance, which also increases the "Q" of the speaker and the bass amplitude at the driver's resonant frequency.
Also, most of the peakiness of low Q, high sensitivity drivers is generated by the cone itself, but some peakiness can also come from the back wave reflected off the back of the enclosure and through the cone, especially in shallow enclosures that don't have dense stuffing. I've found that a piece of Armstrong 420 contractor series fiberglass ceiling panel glued to the rear wall behind the driver will absorb more high frequency reflections than convoluted foam, loosely stuffed pillow stuffing, or even fiberglass ceiling insulation. This material is about 3/4" thick, and the piece needn't be much larger than the driver itself.