I did a little Google "research" on the Heresy II. Didn't find very much, but did learn the woofer impedance is low enough (and the midrange is high enough) to depress the bass (and enhance the mirange) a few dB with moderate-damping SET amps. It is specified to be a nominal 8-ohm speaker, but will do better on a 4-ohm amp output. So I support Tom-s's idea - it will get you closer to the Bryston tonal character, though not all the way.
Even with high-damping amps like the Bryston the Heresy II is a little weak in the bass, based on the measurements I've seen. It's a compromise design, to get the box small without losing midband efficiency. So you might want a greater adjustment range. An equalization similar to a baffle step corrector, tuned to match the 700Hz crossover, would allow greater boost. You could implement it at the input of the Stereomour, (crude but effective) or in an external box (more elegant but more work). It's only a couple of small caps and resistors. If you are interested, I'll work up some circuit ideas.